By Douglas MacMillan 

Yahoo Inc. on Thursday announced a plan to buy back $2 billion of its own shares with proceeds from the recent public offering of Alibaba Group Holding Ltd.

The buybacks are part of Chief Executive Marissa Mayer's broader effort to unlock value from Yahoo's Asian assets and return cash to shareholders. The company has repurchased $9.7 billion in stock since 2013 and earlier this year unveiled a plan to spin off its remaining stake in Alibaba.

The new plan expires March 31, 2018. Yahoo said about $726 million remains available under a repurchase program announced in November 2013.

The additional buybacks bolster Ms. Mayer's defense against activist investor Starboard Value LP, who took a position in Yahoo last year and argued the CEO has diminished investor value by overspending on acquisitions. The investor reiterated some of its concerns in a letter to Ms. Mayer earlier this month, urging the company to cut costs, spin off its stake in Yahoo Japan and buy back up to $4 billion in shares.

Friday is the deadline for shareholders to submit nominees for the board of Yahoo, effectively the last day that a proxy fight can be launched.

A spokesman for Starboard wasn't immediately able to comment Thursday. The firm's latest letter didn't indicate whether the activist may seek a leadership change, a threat it had made before the Alibaba spinoff announcement.

Starboard has voiced the loudest objection to Ms. Mayer in the more than two years since she took the reins at Yahoo. During that period, the Internet portal has failed to boost its core advertising business and struggled to reinvent itself in an age of social networks and mobile computing.

Yahoo reaped more than $5 billion from Alibaba's record-setting IPO last fall, in addition to its sale of $7.6 billion in shares of the Chinese e-commerce giant in 2012.

Yahoo had about $8 billion in cash and short-term securities as of Dec. 31.

Buybacks diminish the amount Ms. Mayer could spend on large acquisitions. Last year, she spent more than $800 million to acquire two companies: mobile ad analytics company Flurry and video-ad service BrightRoll.

In after-hours trading Thursday, Yahoo's shares rose 2.2% to $45.45.

David Benoit contributed to this article.

Write to Douglas MacMillan at douglas.macmillan@wsj.com

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