SELLAS Announces Pricing of $16.2 Million Registered Direct Offering

SELLAS Announces Pricing of $16.2 Million

Registered Direct Offering

YORK, December 13, 2020 -- SELLAS Life Sciences Group, Inc. (Nasdaq: SLS) ("SELLAS" or the "Company"),

a late-stage clinical biopharmaceutical company focused on the development of novel cancer immunotherapies for a broad range of

cancer indications, today announced that it has entered into a share purchase agreement with institutional investors to purchase

approximately $16.2 million of its common stock in a registered direct offering at a purchase price of $7.00 per share.

Under the terms of the share purchase agreement, SELLAS has

agreed to sell 2,320,000 shares of its common stock. The gross proceeds to the Company from the registered direct offering are

expected to be approximately $16.2 million before deducting the placement agent's fees and other offering expenses. The registered

direct offering is expected to close on or about December 16, 2020, subject to the satisfaction of customary closing conditions.

Maxim Group LLC is acting as the exclusive lead placement agent

in connection with the offering.

The shares of common stock are being offered pursuant to a shelf

registration statement on Form S-3 (File No. 333-233869) previously filed and declared effective by the Securities and

Exchange Commission (SEC).

This press release does not constitute an offer to sell or the

solicitation of an offer to buy, nor will there be any sales of these shares of common stock in any jurisdiction in which such

offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of such jurisdiction.

A prospectus supplement relating to the shares of common stock will be filed by SELLAS with the SEC. When available, copies of

the prospectus supplement relating to the registered direct offering, together with the accompanying prospectus, can be obtained

at the SEC's website at www.sec.gov or from Maxim Group LLC, 405 Lexington Avenue, New York, NY 10174, Attention: Syndicate

Department, or via email at syndicate@maximgrp.com or telephone at (212) 895-3745.

About SELLAS Life Sciences Group, Inc.

SELLAS is a late-stage clinical biopharmaceutical company focused

on novel cancer immunotherapeutics for a broad range of cancer indications. SELLAS' lead product candidate, galinpepimut-S

(GPS), is licensed from Memorial Sloan Kettering Cancer Center and targets the Wilms Tumor 1 (WT1) protein, which is present in

an array of tumor types. GPS has potential as a monotherapy or in combination to address a broad spectrum of hematologic malignancies

and solid tumor indications. SELLAS' second product candidate, nelipepimut-S (NPS), is a HER2-directed cancer immunotherapy

with potential for the treatment of patients with early stage breast cancer with low to intermediate HER2 expression, otherwise

known as HER2 1+ or 2+, which includes triple negative breast cancer patients, following standard of care.

more information on SELLAS, please visit www.sellaslifesciences.com.

Forward-Looking Statements

This press release contains forward-looking statements. All

statements other than statements of historical facts are "forward-looking statements," including those relating to

future events. In some cases, forward-looking statements can be identified by terminology such as "plan," "expect,"

"anticipate," "may," "might," "will," "should," "project,"

"believe," "estimate," "predict," "potential," "intend," or "continue"

and other words or terms of similar meaning. These statements include, without limitation, statements related to our ability to

close the offering and the gross proceeds from the offering. These forward-looking statements are based on current plans, objectives,

estimates, expectations and intentions, and inherently involve significant risks and uncertainties. Actual results and the timing

of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties,

which include, without limitation, risks and uncertainties set forth under the caption "Risk Factors" in SELLAS'

Annual Report on Form 10-K filed on March 13, 2020 and in its other SEC filings. Other risks and uncertainties of which SELLAS

is not currently aware may also affect SELLAS' forward-looking statements and may cause actual results and the timing of

events to differ materially from those anticipated. The forward-looking statements herein are made only as of the date hereof.

SELLAS undertakes no obligation to update or supplement any forward-looking statements to reflect actual results, new information,

future events, changes in its expectations or other circumstances that exist after the date as of which the forward-looking statements

SELLAS Life Sciences Group, Inc.