Witt spent most of his first 10 seasons with the Washington Capitals but was dealt to the Nashville Predators at the March trade deadline. He asked at the start of the season to be moved because he didn't want to part of a rebuilding process with the Capitals.
"We were looking for competitors, and today we signed one of the fiercest in the league in Brendan Witt," Islanders general manager Neil Smith said. "Our coach Ted Nolan holds Brendan in the highest regard and was very interested in bringing him to Long Island, and I know Brendan was extremely excited about the prospect of playing for Ted."
Witt has played 642 regular-season games in the NHL, all but 17 with the Capitals, who chose him with the No. 11 pick in the 1993 draft. The 6-foot-2, 220-pound, hard-hitting defenseman has 20 career goals, 66 assists and 1,103 penalty minutes.
"From the beginning, the Islanders were at the top of the list of teams I wanted to sign with," Witt said. "This is a franchise with an incredible tradition and I'm excited to be part of the new era on Long Island. I appreciate the faith Neil Smith has put in me and I'm looking forward to being coached by Ted Nolan.
"We're going to be a team that Islander fans can be proud of."
Smith and Nolan were hired together during last month's Stanley Cup finals. The Islanders missed the playoffs for the first time in four seasons after going seven years without a postseason appearance.
New York hasn't won a playoff series since 1993.
"After battling against Brendan for 10 seasons when he was with the Capitals, our players and our fans know how great it is to have him on our side," said Smith, who also spent time as New York Rangers GM. "He never takes a shift off. There's no question we're a much tougher matchup with Brendan on our blue line."
Witt, 31, had one goal and 10 assists in 55 games before the Capitals sent him to Nashville. He posted three assists with the Predators through the remainder of the regular season and was scoreless in their five-game loss to San Jose in the first round of the playoffs.