Einstein Noah Restaurant Group announced on Monday that it reached an agreement to be acquired for $20.25 per share in cash, or approximately $374 million.
In afternoon trade on Monday, shares of Einstein were up more than 50% to about $20.15.
David Einhorn's hedge fund, Greenlight Capital, is the largest stockholder of Einstein Noah Restaurant Group and owns more than 35% of its shares, according to the company.
According to data from Bloomberg, Einhorn owns more than 13 million shares of Einstein, a stake worth more than $270 million at current prices.
Einhorn called the deal a "win-win for all parties."