[SCHEDULE 13D] Waystar Holding Corp. SEC Filing

Rhea-AI Filing Summary

Advent International affiliates filed a Schedule 13D on Waystar Holding Corp. (WAY), reporting beneficial ownership of 11,059,899 shares of Common Stock, representing 5.8% of the class.

The stake relates to Waystar’s acquisition of Iodine Software, for which the issuer paid $458,598,270 in cash and issued 16,639,920 shares; closing was announced on October 1, 2025. The calculation of ownership references 174,146,070 shares outstanding as of September 11, 2025, increased by the merger share issuance.

Affiliates entered a lock-up for 18 months after the Closing Date, and Advent LP gained the right to nominate one director while it and permitted transferees hold at least 5%; the Board was expanded to 13 and Lauren Young was appointed. After 18 months, Advent may initiate up to two short-form demand registrations and/or shelf take-downs, has piggyback rights, and Waystar will, upon request, amend its shelf to include Advent’s merger shares.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

Waystar Holding Corp.

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

946784105

(CUSIP Number)

Amanda McGrady Morrison
Advent International, L.P., Prudential Tower, 800 Boylston Street
Boston, MA, 02199-8069
617-951-0555

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

10/01/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SCHEDULE 13D

CUSIP No. 946784105

1

Name of reporting person

ADVENT INTERNATIONAL, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

OO

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 11,059,899.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 11,059,899.00

11

Aggregate amount beneficially owned by each reporting person

11,059,899.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

5.8 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus 424B4 ("Prospectus") filed with the Securities and Exchange Commission ("SEC") on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger (as defined in Item 3).

SCHEDULE 13D

CUSIP No. 946784105

1

Name of reporting person

Advent International GP, LLC

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

OO

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 11,059,899.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 11,059,899.00

11

Aggregate amount beneficially owned by each reporting person

11,059,899.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

5.8 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger.

SCHEDULE 13D

CUSIP No. 946784105

1

Name of reporting person

AIO Holdings L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

OO

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 11,059,899.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 11,059,899.00

11

Aggregate amount beneficially owned by each reporting person

11,059,899.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

5.8 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The percent of class is calculated based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger.

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, par value $0.01 per share
(b)	Name of Issuer: Waystar Holding Corp.
(c)	Address of Issuer's Principal Executive Offices: 1550 Digital Drive, #300, Lehi, UTAH , 84043.
Item 2.	Identity and Background
(a)	This statement on Schedule 13D ("Schedule 13D") is filed by the entities listed below, all of whom together are referred to herein as the "Reporting Persons": (i) Advent International, L.P. (f/k/a Advent International Corporation) a Delaware limited partnership ("Advent LP") (ii) Advent International GP, LLC, a Delaware limited liability company ("Advent GP LLC") (iii) AIO Holdings L.P. a Delaware limited partnership ("AIO")
(b)	The principal business address of each of the Reporting Persons is c/o Advent International, L.P., Prudential Tower, 800 Boylston Street, Suite 3300 Boston, MA 02199-8069.
(c)	The principal business of the Reporting Persons is to hold, and/or serve as an advisor with respect to, global equity investments.
(d)	During the last five years, none of the Reporting Persons or, to the knowledge of the Reporting Persons, any of the persons listed on Exhibit 99.1 attached hereto have been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).
(e)	During the last five years, none of the Reporting Persons or, to the knowledge of the Reporting Persons, any of the persons listed on Exhibit 99.1 attached hereto have been party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.
(f)	The name, citizenship, business address, principal business occupation or employment of Advent LP are set forth in Exhibit 99.1 attached hereto.
Item 3.	Source and Amount of Funds or Other Consideration
	Merger Agreement On July 23, 2025, the Issuer entered into an Agreement and Plan of Merger (the "Merger Agreement") by and among the Issuer, Morton Merger Sub 1, Inc., a Delaware corporation and a wholly-owned subsidiary of the Company ("Reverse Merger Sub"), Isotope Holding, LLC, a Delaware limited liability company and a wholly-owned subsidiary of the Issuer ("Forward Merger Sub"), Iodine Software Holdings, Inc. ("Iodine"), Iodine Software Parent, LLC ("Parent") and Shareholder Representative Services LLC, as the equityholder representative, pursuant to which the Issuer will acquire Iodine through a sequence of mergers with Reverse Merger Sub and Forward Merger Sub, with Forward Merger Sub as the surviving company of such mergers (the "Merger"). The Merger was unanimously approved by each of the board of directors of the Issuer (the "Board") and the board of directors of Iodine and was also approved by Parent, its sole equityholder as of July 23, 2025. The consideration paid by the Issuer was comprised of approximately $458,598,270 in cash consideration and an aggregate of 16,639,920 shares of Common Stock issued to certain equityholders of Parent. On October 1, 2025 (the "Closing Date"), the Issuer announced the closing of the Merger. The foregoing description of the Merger Agreement is qualified by reference to such agreement, which is attached hereto as Exhibit 99.2 and incorporated by reference herein.
Item 4.	Purpose of Transaction
	The Reporting Persons hold the reported securities for investment purposes, but intend to review their investment in the Issuer on an ongoing basis and may seek to increase or decrease their position in the Issuer by purchasing additional Common Stock and/or other equity, debt, notes or other financial instruments related to the Issuer (including, for this purpose, any rights or securities exercisable or convertible into securities of the Issuer) or by selling or otherwise disposing of some or all of the reported Common Stock, other equity, debts, notes or other financial instruments (which dispositions may include distributions of some or all of such securities to such Reporting Person's respective members, stockholders, partners or beneficiaries, as applicable, transferring Issuer securities to affiliated transferees, or the entry into a total return swap, asset swap or repurchase transactions) from time to time, in each case, in open market or private transactions, block sales or otherwise. The Reporting Persons may also engage from time to time in ordinary course transactions with financial institutions with respect to the securities described herein. Any such transactions that any of the Reporting Persons or their affiliates may pursue may be made at any time and from time to time without prior notice and will depend on a variety of factors, including, without limitation, the price and availability of the Issuer's securities or other financial instruments; an ongoing evaluation of the Issuer's business, financial condition, operations and prospects; general industry and economic conditions; the securities markets in general; tax considerations; the Reporting Persons' or such affiliates' trading and investment strategies, other investment and business opportunities, applicable legal and/or contractual restrictions and liquidity requirements; and other factors deemed relevant by such Reporting Persons and such affiliates.
Item 5.	Interest in Securities of the Issuer
(a)	The information set forth in rows (11) and (13) of the cover pages of this Schedule 13D is incorporated by reference into this Item 5(a). The reported securities are directly held by AIO, which represents approximately 5.8% of the outstanding Common Stock based on 174,146,070 shares of Common Stock outstanding as of September 11, 2025, as reported by the Issuer in its Prospectus filed with the SEC on September 11, 2025, as such amount is increased by the 16,639,920 shares of Common Stock issued in connection with closing of the Merger. Advent LP is the general partner of AIO and Advent GP LLC is the general partner of Advent LP. Each of Advent LP and Advent GP LLC may be deemed to have voting and dispositive power over the shares directly held by AIO. Pursuant to Rule 13d-4 of the Act, the Reporting Persons declare that filing this statement shall not be construed as an admission that any of the Reporting Persons are, for the purposes of Section 13(d) and/or Section 13(g) of the Act, the beneficial owner of any securities covered by this Schedule 13D, and such beneficial ownership is expressly disclaimed by the Reporting Persons.
(b)	The information set forth in rows (7) through (10) of the cover pages and Item 5(a) of this Schedule 13D is incorporated by reference into this Item 5(b).
(c)	Except as set forth in Item 3 of this Schedule 13D, the Reporting Persons have not effected any transactions in Common Stock within last 60 days.
(d)	Not applicable.
(e)	Not applicable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	Stockholder and Lock-Up Agreements In connection with entry into the Merger Agreement, on July 23, 2025, the Issuer entered into a Stockholder and Lockup Agreement (the "Stockholder and Lockup Agreement"), effective upon the Closing Date, with certain affiliates of Advent LP, the chief executive officer of Iodine (the "Iodine CEO"), and certain other equityholders of Parent which includes, among other things, (i) restricting the transfer of the Common Stock to be received by them in connection with the Merger for a period of 18 months after the Closing Date for Advent LP and the Iodine CEO and (ii) that on the terms and subject to the conditions set forth in the Stockholder and Lockup Agreement, from and after the Closing Date, Advent LP will be entitled to nominate a director to be appointed to the Board until such time as Advent LP and their permitted transferees no longer beneficially own at least 5% of the then-outstanding shares of Common Stock. On the Closing Date, the Issuer amended its stockholders agreement to increase the size of the Board from twelve (12) directors to thirteen (13) directors and appointed Lauren Young pursuant to Advent LP's nomination right (such amendment, the "Existing Stockholders Agreement Amendment"). Joinder to Registration Rights Agreement In connection with entry into the Merger Agreement, on July 23, 2025, the Issuer entered into a Joinder Agreement to the Amended and Restated Registration Rights Agreement, dated as of June 10, 2024 (the "Joinder Agreement"), with Advent LP, which provides for, among other things, that 18 months after the Closing Date, (i) the right to initiate up to two short-form demand registrations and/or shelf take-downs, (ii) certain piggyback registration rights of Advent as a holder thereunder and (iii) an obligation of the Issuer, upon the request of Advent, to amend its existing shelf registration statement to include shares of Common Stock received by Advent LP in the Merger. The foregoing descriptions of the Stockholder and Lockup Agreement, Existing Stockholders Agreement Amendment, and Joinder Agreement are qualified by reference to such agreements, which are attached hereto as Exhibits 99.3, 99.4 and 99.5, respectively, and incorporated by reference herein.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 24 Joint Filing Agreement, dated as of October 3, 2025, by and among the Reporting Persons. Exhibit 99.1 Directors and Executive Officers of the Reporting Persons Exhibit 99.2 Agreement and Plan of Merger, dated as of July 23, 2025, by and among Waystar Holding Corp., Morton Merger Sub 1, Inc., Isotope Holding, LLC, Iodine Software Holdings, Inc., Iodine Software Parent, LLC and Shareholder Representative Services LLC, as the equityholder representative (incorporated by reference to Exhibit 2.1 to the Form 8-K filed by Waystar Holding Corp. on July 23, 2025). Exhibit 99.3 Stockholder and Lockup Agreement among Waystar Holding Corp., AIO Holdings LP, William Chan and the other parties named therein, dated as of July 23, 2025 (incorporated by reference to Exhibit 10.1 to the Form 8-K filed by Waystar Holding Corp. on July 23, 2025). Exhibit 99.4 Amendment No. 2 to the Stockholders Agreement among Waystar Holding Corp. and the other parties named therein, dated as of July 23, 2025 (incorporated by reference to Exhibit 10.2 to the Form 8-K filed by Waystar Holding Corp. on July 23, 2025). Exhibit 99.5 Joinder Agreement to Amended and Restated Registration Rights Agreement, dated as of July 23, 2025 (incorporated by reference to Exhibit 10.3 to the Form 8-K filed by Waystar Holding Corp. on July 23, 2025).

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

ADVENT INTERNATIONAL, L.P.

Signature:	/s/ Neil Crawford
Name/Title:	Neil Crawford / Vice President of Finance
Date:	10/03/2025

Advent International GP, LLC

Signature:	/s/ Neil Crawford
Name/Title:	Neil Crawford / Vice President of Finance
Date:	10/03/2025

AIO Holdings L.P.

Signature:	/s/ Neil Crawford
Name/Title:	Neil Crawford / Treasurer and Secretary
Date:	10/03/2025

Comments accompanying signature:

Advent International, L.P., By: Advent International GP, LLC, its General Partner. AIO Holdings, L.P., By: Advent International, L.P., its General Partner, By: Advent International GP, LLC, its General Partner.

FAQ

How many Waystar (WAY) shares does Advent report owning?

Advent affiliates report 11,059,899 shares of Common Stock, or 5.8% of the class.

What share count underlies the 5.8% ownership for WAY?

The calculation references 174,146,070 shares outstanding as of September 11, 2025, increased by 16,639,920 shares issued at merger closing.

Which Advent entity directly holds the WAY shares?

AIO Holdings L.P. directly holds the reported shares; Advent LP and Advent GP LLC may be deemed to have voting and dispositive power.

What were the merger terms tied to Advent’s WAY stake?

Waystar’s Iodine acquisition included $458,598,270 in cash and 16,639,920 shares issued; closing was announced on October 1, 2025.

Are Advent’s WAY shares subject to transfer restrictions?

Yes. Affiliates agreed to a lock-up for 18 months after the Closing Date.

Does Advent have board rights at Waystar (WAY)?

Yes. Advent LP may nominate one director while it and permitted transferees own at least 5%. The Board was increased to 13 and Lauren Young was appointed.

What registration rights does Advent have for its WAY shares?

After 18 months, Advent may initiate up to two short-form demand registrations and/or shelf take-downs, has piggyback rights, and Waystar will amend its shelf upon request to include these shares.