13D Filing: Kim D Blickenstaff and Tandem Diabetes Care Inc (NASDAQ:TNDM)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE
13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 2)*

Tandem
Diabetes Care, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

875372 104

(CUSIP
Number)

Kim D. Blickenstaff

President and Chief Executive Officer

Tandem Diabetes Care, Inc.

11045 Roselle Street

San
Diego, California 92121

(858) 366-6900

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

March 28, 2017

(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.  ☐

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See
section 240.13d-7 for other parties to whom copies are to be sent.

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).

EXPLANATORY NOTE: This Amendment No. 2 to Schedule 13D (the Amendment) is being filed
on behalf of Kim D. Blickenstaff (Mr. Blickenstaff) and the Kim Blickenstaff Revocable Trust dated April 15, 2010, which Mr. Blickenstaff is trustee (the Trust, and together with Mr. Blickenstaff, the
Reporting Persons) to amend the statement on Schedule 13D relating to the common stock, $0.001 par value per share (the Common Stock), of Tandem Diabetes, Inc., a Delaware corporation (the Issuer), as filed with
the Securities and Exchange Commission (the SEC) on November 26, 2013 (the Original Schedule 13D) and amended by the filing of Amendment No. 1 to Schedule 13D, as filed with the SEC on November 27, 2013
(Amendment No. 1, together with the Original Schedule 13D, the Previously Amended Schedule 13D). This Amendment is being filed to report the Trusts purchase of 1,600,000 shares of Common Stock in the Issuers public
offering that closed on March 28, 2017. The Previously Amended Schedule 13D is hereby amended and supplemented as detailed below and, except as amended and supplemented hereby, the Previously Amended Schedule 13D remains in full force and
effect. All capitalized terms not otherwise defined herein shall have the meaning ascribed to the terms in the Previously Amended Schedule 13D.

Item 3 of the Previously Amended Schedule
13D is hereby amended and restated in its entirety to read as follows:

Mr. Blickenstaff has the right to acquire 1,518,468 shares of Common
Stock upon the exercise of vested and unvested options that were granted without consideration in exchange for employment by the Issuer. Of this amount, 1,208,059 shares are issuable upon the exercise of options that are exercisable within 60 days
of the date hereof.

The Trust owns 2,007,939 shares of Common Stock, including (i) 407,939 shares of Common Stock received upon the conversion of the
Issuers preferred stock at the time of the Issuers initial public offering, and (ii) 1,600,000 shares of Common Stock purchased on March 28, 2017 in the Issuers firm commitment underwritten offering (the Public
Offering) at the public offering price of $1.25 per share. The source of funds for the purchase of the shares of Common Stock in the Public Offering was trust funds. The Trust also holds warrants to purchase in the aggregate 135,633 shares of
Common Stock.

Item 5 of the Previously Amended Schedule 13D is hereby
amended and restated in its entirety to read as follows:

The Trust is the beneficial owner of an aggregate of 2,143,572 shares of Common Stock,
representing 4.3% of the outstanding Common Stock. The Trusts ownership is comprised of (i) 2,007,939 shares of Common Stock held by the Trust, and (ii) 135,633 shares of Common Stock underlying warrants held by the Trust.

All percentages in this Item 5 relating to beneficial ownership of Common Stock are based on
49,836,070 shares of Common Stock issued and outstanding as of March 28, 2017, as reported by the Issuer to the Reporting Persons.

The outstanding option table in Item 6 of the Previously Amended Schedule 13D is hereby amended and restated in its entirety to read as follows:

The following table summarizes the terms of all outstanding options that Issuer has granted to Mr. Blickenstaff pursuant to the Issuers 2006 Stock
Incentive Plan or the Issuers 2013 Stock Incentive Plan.

The third paragraph following the outstanding option table in Item 6 of the Previously Amended Schedule
13D is hereby amended and restated in its entirety to read as follows:

On March 1, 2017, in connection with the Public Offering,
the Reporting Persons, together with other officers, directors and stockholders of the Issuer, entered into a Lock-Up Agreement. Pursuant to such Lock-Up Agreement, the
Reporting Persons agreed that they will not sell, transfer or dispose of, directly or indirectly, any of the Common Stock or securities convertible into or exercisable or exchangeable for Common Stock for a period of 90 days after the date of the
final prospectus for the Public Offering. This summary description does not purport to be complete, and is qualified in its entirety by the Form of Lock-Up Agreement attached as Exhibit C to the Form of
Underwriting Agreement filed as Exhibit 1.1 to the Registration Statement on Form S-1 filed with the SEC on March 22, 2017, which is incorporated herein by reference.

SIGNATURE

After reasonable inquiry and to the best of the undersigneds knowledge and belief, the undersigned certifies that the information set
forth in this statement is true, complete and correct.

Date: April 4, 2017.