Highland Capital Management Reveals Investment In Relypsa (RLYP)

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13G

(Rule 13d-102)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO

RULES 13d-1(b), (c) AND (d) AND AMENDMENTS THERETO FILED

PURSUANT TO RULE 13d-2

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Relypsa, Inc.

(Name of Issuer)

Common stock, par value $0.001 per share

(Title of Class of Securities)

75931106

(CUSIP Number)

July 27, 2016

(Date of Event Which Requires Filing of this Statement)

Check the
appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule
13d-1(b)

¨ Rule 13d-1(c)

¨ Rule 13d-1(d)

SCHEDULE 13G

This Schedule 13G (this Schedule 13G) is being filed on behalf of Highland Small-Cap Equity Fund, a series of Highland Funds II, a
Massachusetts business trust (the Small-Cap Fund), Highland Capital Management Fund Advisors, L.P., a Delaware limited partnership (Highland Fund Advisors), Strand Advisors XVI, Inc., a Delaware corporation (Strand
XVI), NexPoint Credit Strategies Fund, a Delaware statutory trust (the Credit Fund), NexPoint Advisors, L.P., a Delaware limited partnership (NexPoint), NexPoint Advisors GP, LLC, a Delaware limited liability company
(NexPoint GP), Highland Select Equity Master Fund, L.P., a Bermuda limited partnership (the Select Fund and collectively with the Small-Cap Fund and the Credit Fund, the Funds), Highland Select Equity Fund GP,
L.P., a Delaware limited partnership (Select GP), Highland Select Equity GP, LLC, a Delaware limited liability company (Select LLC), Highland Capital Management, L.P., a Delaware limited partnership (Highland
Capital), Strand Advisors, Inc., a Delaware corporation (Strand), and James D. Dondero (collectively, the Reporting Persons).

Brad Ross is the President of Strand XVI and James D. Dondero is the President of NexPoint GP and Strand. Strand XVI is the general partner of
Highland Fund Advisors. Highland Fund Advisors is the investment advisor to the Small-Cap Fund. NexPoint GP is the general partner of NexPoint. NexPoint is the investment advisor to the Credit Fund. Strand is the general partner of Highland Capital.
Highland Capital is the sole member of Select LLC. Select LLC is the general partner of Select GP. Select GP is the general partner of the Select Fund. This Schedule 13G relates to the Common stock, par value $0.001 per share (the Common
Stock), of Relypsa, Inc., a Delaware corporation (the Issuer), held by the Funds.

Relypsa, Inc.

100 Cardinal Way

Redwood City, CA 94063

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For all Filers:

300 Crescent Court, Suite 700

Dallas, Texas 75201

Common stock, par value $0.001 per share

75931106

If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b), check whether
the person filing is a:

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The Credit Fund may be deemed the beneficial owner of 2,600,000 shares of Common Stock that it holds directly, which consist of 2,600,000
shares of Common Stock receivable upon exercise of presently exercisable call options which expire September 16, 2016 and have an exercise price of $32. NexPoint, as the investment advisor to the Credit Fund, and NexPoint GP, as the general
partner of NexPoint, may be deemed the beneficial owners of the 2,600,000 shares of Common Stock held by the Credit Fund.

The Select Fund
may be deemed the beneficial owner of 3,277,900 shares of Common Stock that it holds directly, which consist of 128,000 shares of Common Stock and 3,149,900 shares of Common Stock receivable upon exercise of presently exercisable call options which
expire September 16, 2016 and have an exercise price of $32. Select GP, as the general partner of the Select Fund, Select LLC, as the general partner of Select GP, Highland Capital, as the sole member of Select LLC, and Strand, as the general
partner of Highland Capital, may be deemed the beneficial owners of the 3,277,900 shares of Common Stock held by the Select Fund.

Mr. Dondero may be deemed the beneficial owner of the 5,914,900 shares of Common Stock held by the Funds, which consist of 165,000 shares
of Common Stock and 5,749,900 shares of Common Stock receivable upon exercise of presently exercisable call options which expire September 16, 2016.

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The Credit Fund, NexPoint and NexPoint GP may be deemed the
beneficial owners of 5.8% of the outstanding shares of Common Stock held by the Credit Fund. This percentage was determined by dividing 2,600,000, the number of shares of Common Stock held directly by the Credit Fund, by 44,747,978, which is the
number of shares of Common Stock outstanding as of May 2, 2016 according to the Issuers Quarterly Report on Form 10-Q filed with the Securities Exchange Commission on May 4, 2016.

The Select Fund, Select GP, Select LLC, Highland Capital and Strand may be deemed the beneficial owner of 7.3% of the outstanding Common
Stock. This percentage was determined by dividing 3,277,900, the number of shares of Common Stock held directly by the Select Fund, by 44,747,978, which is the number of shares of Common Stock outstanding as of May 2, 2016 according to the
Issuers Quarterly Report on Form 10-Q filed with the Securities Exchange Commission on May 4, 2016.

Mr. Dondero may be
deemed the beneficial owner of 13.2% of the outstanding Common Stock. This percentage was determined by dividing 5,914,900, the number of shares of Common Stock held directly by the Funds, by 44,747,978, which is the number of shares of Common Stock
outstanding as of May 2, 2016 according to the Issuers Quarterly Report on Form 10-Q filed with the Securities Exchange Commission on May 4, 2016.

The Credit Fund has the sole power to vote and dispose of the 2,600,000 shares
of Common Stock that it holds directly. NexPoint and NexPoint GP have the shared power to vote and dispose of the 2,600,000 shares of Common Stock held by the Credit Fund.

The Select Fund has the sole power to vote and dispose of the 3,277,900 shares of Common Stock that it holds directly. Select GP, Select LLC,
Highland Capital and Strand have the shared power to vote and dispose of the 3,277,900 shares of Common Stock held by the Select Fund.

Mr. Dondero has the the shared power to vote and dispose of the 5,914,900 shares of Common Stock held by the Funds.

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Inapplicable.

Inapplicable.

Inapplicable.

Inapplicable.

Inapplicable.

By signing below each of the Reporting Persons certifies that, to the
best of such persons knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired
and are not held in connection with or as a participant in any transaction having that purpose or effect.

Joint Filing Agreement, dated August 8, 2016, by and among the
Small-Cap Fund, Highland Fund Advisors, Strand XVI, the Credit Fund, NexPoint, NexPoint GP, the Select Fund, Select GP, Select LLC, Highland Capital, Strand and James D. Dondero.

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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.

Date: August 8, 2016

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