Ristolainen (6-foot-4, 225 pounds) was the Sabres' highest draft selection since they chose forward Thomas Vanek fifth in 2003.
"We felt that it was important to get Rasmus under contract quickly so that we could start the development process right away," general manager Darcy Regier said in a statement. "He's a very mature defenseman for his age (19 in October) and will have every chance to compete for a roster spot in training camp."
Ristolainen played last season for TPS in his native Finland, with 15 points (three goals) and 32 penalty minutes in 52 games. He was a member of Finland at the 2013 IIHF World Junior Championship.