Activist shareholder Starboard Value LP, which has acquired a stake in Yahoo! Inc., said it wants Chief Executive Officer Marissa Mayer to cut costs, halt acquisitions and explore a combination with AOL Inc.

In a letter today to Mayer, the New York-based shareholder said it sees opportunities for Yahoo to “unlock tremendous value.” The letter, signed by Starboard CEO Jeffrey Smith, said that the Sunnyvale, California-based Web portal is currently “deeply undervalued relative to the sum of its parts.”

Starboard’s letter follows the initial public offering last week of Alibaba Group Holding Ltd., in which Yahoo owns a stake and which had driven up much of the Silicon Valley company’s value. As investors took gains on Yahoo after Alibaba’s IPO, the true worth of the Web portal’s core online-advertising business was laid bare. Yahoo was worth less than the value of its Asian assets following Alibaba’s IPO.

This isn’t Yahoo’s first encounter with an activist investor. Daniel Loeb’s Third Point LLC took a stake in Yahoo in 2011, when the fund bought a 5.2 percent stake and urged the board to resign. Loeb forced the ouster of former Yahoo CEO Scott Thompson and took a seat on the board, and later sold the stake back to the company.