As filed with the Securities and Exchange Commission on January 29, 2020.

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

Under

The Securities Act of 1933

ALECTOR, INC.

(Exact name of Registrant as specified in its charter)

131 Oyster Point Blvd. Suite 600

South San Francisco, California 94080

415-231-5660

(Address, including zip code, and telephone number, including area code, of Registrant’s principal executive offices)

Arnon Rosenthal, Ph.D.

Chief Executive Officer

Alector, Inc.

131 Oyster Point Blvd. Suite 600

South San Francisco, California 94080

415-231-5660

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after the effective date of this registration statement.

If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  ☐

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☒ 333-236094

If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☒

CALCULATION OF REGISTRATION FEE

This registration statement shall become effective upon filing with the Securities and Exchange Commission in accordance with Rule 462(b) of the Securities Act of 1933, as amended.

EXPLANATORY NOTE AND INCORPORATION BY REFERENCE

Alector, Inc., a Delaware corporation (“Alector”), is filing this registration statement with the Securities and Exchange Commission pursuant to Rule 462(b) under the Securities Act of 1933, as amended. This registration statement relates to the public offering of securities contemplated by the registration statement on Form S-1 (File No. 333-236094) originally filed on January 27, 2020, as amended, or the Prior Registration Statement, and which the Securities and Exchange Commission declared effective on January 29, 2020.

Alector is filing this registration statement for the sole purpose of increasing by 1,552,500 shares the number of shares of its common stock, par value $0.0001 per share, to be registered for sale. The additional shares that are being registered for sale are in an amount and at a price that together represent no more than 20% of the maximum aggregate offering price set forth in the Calculation of Registration Fee table contained in the Prior Registration Statement. The information set forth in the Prior Registration Statement, and all exhibits to the Prior Registration Statement, are hereby incorporated by reference into this registration statement.

The required opinions and consents are listed on the Exhibit Index attached hereto and are filed herewith.

EXHIBIT INDEX

II-1

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of South San Francisco, State of California, on the 29th day of January, 2020.

Pursuant to the requirements of the Securities Act of 1933, as amended, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

II-2

Exhibit 5.1

January 29, 2020

Alector, Inc.

131 Oyster Point Boulevard, Suite 600

South San Francisco, California 94080

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-1 (the “Registration Statement”), filed by Alector, Inc. (the “Company”) with the Securities and Exchange Commission pursuant to Rule 462(b) of the Securities Act of 1933, as amended (“Securities Act”), in connection with the registration under the Securities Act of up to 1,552,500 shares (including up to 202,500 shares issuable upon exercise of an option granted to the underwriters by the Company) of the Company’s common stock, $0.0001 par value per share (the “Shares”), to be issued and sold by the Company. The Registration Statement incorporates by reference the Registration Statement on Form S-1, as amended (Registration No. 333-236094) (the “Prior Registration Statement”), which was declared effective on January 29, 2020, including the prospectus which forms part of the Registration Statement. We understand that the Shares are to be sold to the underwriters for resale to the public as described in the Registration Statement and the Prior Registration Statement and pursuant to an underwriting agreement, substantially in the form filed as an exhibit to the Prior Registration Statement, to be entered into by and among the Company and the underwriters (the “Underwriting Agreement”).

We are acting as counsel for the Company in connection with the sale of the Shares by the Company. In such capacity, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion. In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity with the originals of all documents submitted to us as copies, the authenticity of the originals of such documents and the legal competence of all signatories to such documents.

We express no opinion herein as to the laws of any state or jurisdiction other than the General Corporation Law of the State of Delaware (including the statutory provisions and all applicable judicial decisions interpreting those laws) and the federal laws of the United States of America.

On the basis of the foregoing, we are of the opinion that the Shares to be issued and sold by the Company have been duly authorized and, when such Shares are issued and paid for in accordance with the terms of the Underwriting Agreement, will be validly issued, fully paid and nonassessable.

AUSTIN     BEIJING     BOSTON     BRUSSELS     HONG KONG     LONDON     LOS ANGELES     NEW YORK     PALO ALTO

SAN DIEGO     SAN FRANCISCO     SEATTLE     SHANGHAI     WASHINGTON, DC     WILMINGTON, DE

January 29, 2020

Page 2

We consent to the use of this opinion as an exhibit to the Registration Statement, and we consent to the reference of our name under the caption “Legal Matters” in the prospectus forming part of the Registration Statement.

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in the Registration Statement on Form S-1 filed pursuant to Rule 462(b) of the Securities Act of 1933 of the reference to our firm under the caption “Experts” and to the incorporation by reference of our report dated March 26, 2019, with respect to the consolidated financial statements of Alector, Inc. included in Amendment No. 1 to the Registration Statement (Form S-1 No. 333-236094) and related Prospectus of Alector, Inc. for the registration of its common stock.

/s/ Ernst & Young LLP

San Jose, California

January 24, 2020