Walter, the most valuable player last season for the Bruins' AHL Providence affiliate, had 24 goals and 43 points in 73 games. Nokelainen, 21, posted six goals and 10 assists in 60 games for AHL Bridgeport.
"Ben Walter is a strong addition to our roster and immediately becomes one of our top prospects," Islanders general manager Garth Snow said of the 23-year-old centre. "Our staff loves his two-way play, work ethic and knack for scoring big goals. Ben is a young player who will battle for a position in training camp and gives us excellent depth at an important position."
Walter, the son of former NHL player Ryan Walter, is entering his third professional season after three years at the University of Massachusetts-Lowell. Ben has played 10 games for the Boston Bruins over the last two seasons.
In his last college season, Walter was a finalist for the Hobey Baker Trophy given to the player of the year.
Nokelainen was the Islanders' first-round selection in the 2004 draft. He played 15 games for the Islanders in the 2005-06 season, scoring a goal and adding two assists.