Devils general manager Lou Lamoriello said the Devils had previous discussions with Zajac about locking up the 27-year-old center, but could not complete the final details until the lockout ended.
"He's one of our core leaders and he has been since day one," Lamoriello said.
Lamoriello announced the deal shortly after Zajac scored a power-play goal off a great pass from Ilya Kovalchuk in Wednesday's scrimmage against Albany of the AHL.
"I want to be here," Zajac said. "I believe in this organization. I think it is a first-class organization. It's the environment I want to surround myself on and off the ice, and my family loves New Jersey. We like the people here. We like living here. With all that and the players and coaching staff, I think we have a great chance here to always be successful."
Zajac's deal, which includes a no-trade clause, is a reflection of the new rules on contract limits brokered in the collective bargaining agreement, agreed to by the NHL and NHLPA earlier this month. Those rules allow teams to re-sign their own players to eight-year deals, while limiting all other long-term deals to a maximum of seven years.
Zajac will receive $3.5 million in the first year of the deal and $5 million in the second, with a $1 million bonus in each. He will make $6.5 million in seasons three through six and $5.75 million in each of the last two seasons.
Zajac, who missed the majority of last season recovering from an Achilles injury, was limited to two goals and four assists in 15 regular-season games for the Devils but turned it on during the playoffs.
The former first-round pick (20th overall in 2004) had seven goals and seven assists in 24 playoff games en route to the team's Stanley Cup finals appearance.
"I knew in my gut, I think, I wanted to be here," Zajac said. "When it came down to it, it was an easy decision."
Lamoriello reiterated Wednesday that Parise will be missed.
"You don't replace a Zach Parise," Lamoriello said. "He was an exceptional player, but you have to move on."
Zajac, who is slated to be the team's first-line center, skated on a line with Kovalchuk and Dainius Zubrus in the team's intrasquad scrimmage with Albany on Wednesday.
Zajac's presence in the middle will be key for a Devils squad that will start the season without 2012 Calder Trophy finalist Adam Henrique. The 22-year-old pivot is recovering from a hand injury and, though he skated Wednesday morning, he is expected to miss at least two more weeks.
Information from The Associated Press was used in this report.