Zajac will remain with the Devils in an on- and off-ice player development and consulting role, while also working to grow New Jersey's youth hockey initiatives.
"I took some time off in the summer, and at this age (36) it gets harder and harder to stay at the top, be successful and be competitive," Zajac said. "After just taking time off I really didn't have that desire to keep training, keep pushing myself. I kind of feel like I ran out of a little gas, and being around the family a lot more was starting to have a real big impact on my life. Being home for dinners, being around the kids' sports, involved in school, that was a lot of fun to me and I really love being a part of that. The sacrifices you have to make to continue to play, I didn't have that internal push to really keep going."
Zajac scored 20 points (eight goals, 12 assists) in 46 games last season, including two points (one goal, one assists) in 13 games after the forward was traded to the New York Islanders on April 7. He scored two points (one goal, one assist) in 14 Stanley Cup Playoff games.
He said there were opportunities to continue playing for the Devils and Islanders, but felt retirement was the best option.
"It was a great run, it was a great group of guys," Zajac said of the Islanders. "They have a very competitive team. So from that standpoint it was just a great opportunity and a good experience. [Islanders general manager] Lou [Lamoriello] reached out and I told him the same thing, I wasn't interested. I know for a fact my heart was always to come back and play in New Jersey, but I was ready to take the next step, the next chapter in my life, and that was to retire. At the end of the day, it was a great opportunity and a good team to try to win a Stanley Cup. I'm happy I took that chance to try to get that Cup."
Zajac, who was an unrestricted free agent, was selected by the Devils in the first round (No. 20) of the 2004 NHL Draft and played all but 13 of his 1,037 NHL games with New Jersey.
He is fourth in Devils history in games played (1,024), behind Ken Daneyko (1,283), Martin Brodeur (1,259), and Patrik Elias (1,240). He's third in points (550), tied for third in goals (202) and fifth in assists (348).
"Fifteen years, 1,000 games with one team, that's incredible," New Jersey GM Tom Fitzgerald said. "It really is in today's game. ... Great leader on and off the ice. Just a fantastic teammate."
The Devils will honor Zajac for playing in his 1,000th NHL game on March 10 against his hometown Winnipeg Jets.
"To have the fans in March and celebrate, they've embraced me with open arms from the beginning, through the highs, through the lows," Zajac said. "You play the game and the fans make it that much more special. Looking forward to that night. It'll be special."
NHL.com senior writer Dan Rosen contributed to this report
