The restricted free agent was set to have an arbitration hearing Monday.
Ullmark posted a .915 save percentage and a 2.69 goals-against average in 34 games this past season. He projects to be the club's No. 1 goaltender next year after vastly outplaying veteran Carter Hutton. Both Ullmark and Hutton will be unrestricted free agents next offseason.
The Sabres picked the 27-year-old during the sixth round in 2012.
Sabres general manager Kevyn Adams has been busy Sunday, with the club also avoiding arbitration with forward Sam Reinhart on a one-year, $5.2-million agreement. Victor Olofsson and Lawrence Pilut are the team's remaining RFAs. Casey Mittelstadt is one as well, but he's not on the NHL roster.
Buffalo has $6.5 million left in projected cap space, according to CapFriendly.