Sotheby’s said on Monday that it had ended its fight with the hedge fund mogul Daniel S. Loeb, agreeing to add his three director nominees to its board.

The last-minute settlement – reached a day before investors were scheduled to vote on the board – represents a clear win for Mr. Loeb, the veteran activist investor who has waged a monthslong fight against Sotheby’s in a bid to shake up the 270-year-old auction house.

The fight had become one of the biggest and most intense battles between a company and an activist this year, with each side hurling insults at the other.

Under the terms of their agreement, Sotheby’s will expand its board by three, to 15, to take on the activist’s full slate of nominees: Mr. Loeb himself, the restructuring expert Harry Wilson and the former investment banker Olivier Reza.

And Mr. Loeb’s firm, Third Point, will be allowed to raise its stake to 15 percent from its current level of 10 percent. The hedge fund had sued Sotheby’s in Delaware’s Court of Chancery, arguing that a “poison pill” defense plan that limited him to a 10 percent stake while letting mutual funds acquire up to a 20 percent stake was unfair.