[SCHEDULE 13G] Seaport Therapeutics, Inc. Passive Investment Disclosure (>5%)
PureTech reports 31.2% stake in Seaport
PureTech Health plc, together with its affiliates PureTech Health LLC and PureTech LYT, Inc., reports beneficial ownership of 16,685,013 shares of Seaport Therapeutics, Inc. Voting Common Stock as of June 30, 2026.
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PureTech Health plc, together with its affiliates PureTech Health LLC and PureTech LYT, Inc., reports beneficial ownership of 16,685,013 shares of Seaport Therapeutics, Inc. Voting Common Stock as of June 30, 2026. This represents 31.2% of the class, based on 53,530,550 shares outstanding as of July 27, 2026.
PureTech LYT, Inc. is the record holder of the shares, while PureTech Health LLC and PureTech Health plc indirectly control them through ownership of the subsidiary entities, with shared voting and dispositive power over all 16,685,013 shares.
Key Figures
Shares beneficially owned:16,685,013 sharesPercent of class:31.2%Shares outstanding:53,530,550 shares+2 more
5 metrics
Shares beneficially owned16,685,013 sharesVoting Common Stock beneficially owned as of June 30, 2026
Percent of class31.2%Portion of Seaport Therapeutics Voting Common Stock class
Shares outstanding53,530,550 sharesShares of Voting Common Stock outstanding as of July 27, 2026
Shared voting power16,685,013 sharesShares over which Reporting Persons share voting power
Shared dispositive power16,685,013 sharesShares over which Reporting Persons share dispositive power
Key Terms
beneficial ownership, shared voting power, shared dispositive power, Voting Common Stock, +1 more
5 terms
beneficial ownershipfinancial
"The ownership information presented below represents beneficial ownership of the shares"
Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.
shared voting powerfinancial
"Shared Voting Power 16,685,013.00"
Shared voting power occurs when two or more parties jointly have the right to vote or decide how a block of company shares is cast, like co-owners who must agree before moving a piece of furniture. Investors care because who controls voting rights affects board elections, major corporate decisions and takeover outcomes, and shared control can alter regulatory disclosures and the practical influence any holder has over a company’s direction and value.
shared dispositive powerfinancial
"Shared Dispositive Power 16,685,013.00"
Voting Common Stockfinancial
"Title of class of securities: Voting Common Stock, $0.001 par value per share"
Schedule 13Gregulatory
"form_type": "SCHEDULE 13G"
A Schedule 13G is a formal document that investors file with the government when they acquire a large ownership stake in a company, usually for investment purposes rather than control. It helps keep the public informed about who owns significant parts of a company's shares, which can influence how the company is managed and how investors make decisions. Filing this schedule is important for transparency and understanding the ownership landscape of publicly traded companies.
FAQ
AI-generated questions and answers. How Rhea-AI works. Not financial advice.
What stake does PureTech Health plc report in Seaport Therapeutics (SPTX)?
PureTech Health plc and its affiliates report beneficial ownership of 16,685,013 shares of Seaport Therapeutics Voting Common Stock, representing 31.2% of the outstanding shares based on the issuer’s disclosed share count.
How many Seaport Therapeutics (SPTX) shares are outstanding for this 13G?
The reported 31.2% ownership is calculated using 53,530,550 shares of Seaport Therapeutics Voting Common Stock outstanding as of July 27, 2026, as referenced in the issuer’s Quarterly Report on Form 10-Q.
Which PureTech entity directly holds Seaport Therapeutics (SPTX) shares?
The shares are held of record by PureTech LYT, Inc.. PureTech Health LLC is the sole owner of PureTech LYT, Inc., and PureTech Health plc is the sole member of PureTech Health LLC, giving them indirect beneficial ownership.
What voting and dispositive power do the PureTech entities have over SPTX shares?
The Reporting Persons report 0 shares with sole voting or dispositive power and 16,685,013 shares with shared voting and shared dispositive power, reflecting coordinated control over these Seaport Therapeutics shares.
Who signed the Schedule 13G for the Seaport Therapeutics (SPTX) holdings?
The Schedule 13G was signed by Robert Lyne, Chief Executive Officer, on behalf of each Reporting Person, with signatures dated August 14, 2026, and includes a Joint Filing Agreement as Exhibit 99.1.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
Seaport Therapeutics, Inc.
(Name of Issuer)
Voting Common Stock, $0.001 par value per share
(Title of Class of Securities)
81221K108
(CUSIP Number)
06/30/2026
(Date of Event Which Requires Filing of this Statement)
Check the appropriate box to designate the rule pursuant to which this Schedule is filed:
Rule 13d-1(b)
Rule 13d-1(c)
Rule 13d-1(d)
schemaVersion:
SCHEDULE 13G
CUSIP Number(s):
81221K108
1
Names of Reporting Persons
PureTech Health plc
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
UNITED KINGDOM
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
16,685,013.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
16,685,013.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
16,685,013.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
31.2 %
12
Type of Reporting Person (See Instructions)
CO
SCHEDULE 13G
CUSIP Number(s):
81221K108
1
Names of Reporting Persons
PureTech Health LLC
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
DELAWARE
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
16,685,013.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
16,685,013.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
16,685,013.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
31.2 %
12
Type of Reporting Person (See Instructions)
OO
Comment for Type of Reporting Person: Limited Liability Company
SCHEDULE 13G
CUSIP Number(s):
81221K108
1
Names of Reporting Persons
PureTech LYT, Inc.
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
DELAWARE
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
16,685,013.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
16,685,013.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
16,685,013.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
31.2 %
12
Type of Reporting Person (See Instructions)
CO
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
Seaport Therapeutics, Inc.
(b)
Address of issuer's principal executive offices:
101 Seaport Blvd., Floor 12, Boston MA 02210
Item 2.
(a)
Name of person filing:
Each of the following is hereinafter individually referred to as a "Reporting Person" and collectively as the "Reporting Persons." This statement is filed on behalf of: PureTech Health plc, PureTech Health LLC and PureTech LYT, Inc.
(b)
Address or principal business office or, if none, residence:
The principal business address of each of the Reporting Persons is 6 Tide Street, Suite 400, Boston, MA 02210
(c)
Citizenship:
PureTech Health plc is a public limited company organized under the laws of the United Kindom. PureTech Health LLC is a limited liability company and PureTech LYT, Inc. is a corporation, both organized under the laws of the State of Delaware.
(d)
Title of class of securities:
Voting Common Stock, $0.001 par value per share
(e)
CUSIP Number(s):
81221K108
Item 3.
If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)
Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)
Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)
Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)
Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)
An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)
An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)
A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)
A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)
A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)
A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J),
please specify the type of institution:
(k)
Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.
Ownership
(a)
Amount beneficially owned:
The ownership information presented below represents beneficial ownership of the shares of Voting Common Stock (the "Common Stock") as of June 30, 2026, based upon 53,530,550 shares of Common Stock outstanding as of July 27, 2026, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on August 3, 2026.
PureTech LYT, Inc. is the record holder of the Common Stock reported herein. PureTech Health LLC is the sole owner of PureTech LYT, Inc., and PureTech Health plc is the sole member of PureTech Health LLC, and has the power to direct the voting and disposition of securities held by PureTech LYT, Inc. and PureTech Health LLC. As such, PureTech Health LLC and PureTech Health plc may be deemed to beneficially own all of the shares of Common Stock held directly by PureTech LYT, Inc.
(b)
Percent of class:
31.2%
(c)
Number of shares as to which the person has:
(i) Sole power to vote or to direct the vote:
0
(ii) Shared power to vote or to direct the vote:
16,685,013
(iii) Sole power to dispose or to direct the disposition of:
0
(iv) Shared power to dispose or to direct the disposition of:
16,685,013
Item 5.
Ownership of 5 Percent or Less of a Class.
Not Applicable
Item 6.
Ownership of more than 5 Percent on Behalf of Another Person.
Not Applicable
Item 7.
Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
Not Applicable
Item 8.
Identification and Classification of Members of the Group.
Not Applicable
Item 9.
Notice of Dissolution of Group.
Not Applicable
Item 10.
Certifications:
Not Applicable
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.
