Slater called the extension "humbling" on Wednesday but declined to go into details. The Patriots don't announce contract extensions.
The deal includes $3 million in guaranteed money, including $2 million up front in the form of a signing bonus, plus his 2015 base salary of $1 million. The deal also includes a base salary of $1 million for 2016, and incentives each season of $100,000, which Slater would earn if he makes the Pro Bowl in either season.
If Slater hits both incentives, the deal would reach a maximum value of $4.2 million.
Slater, originally selected by the team out of UCLA in the fifth round of the 2008 draft, has been named to the Pro Bowl on three occasions and is regarded as one of the best coverage players in the NFL. He has played in 98 regular-season games for the Patriots, totaling 99 tackles, while also serving as a situational wide receiver.
The 29-year-old also is highly regarded within the Patriots organization for his leadership.
Coach Bill Belichick wouldn't confirm the extension Wednesday but did praise Slater as a player during his weekly news conference.
"Matt does a lot of things for us," Belichick said. "Obviously he's a very good player on the field, professional, works hard. [He] does a great job with all the players -- younger players -- but also his peers. He's very well-respected in the locker room, not just by the players but by the entire organization by the way he goes about his job, how dependable he is, his work ethic, his attitude, his toughness. He's really such a model player. I'd love to have 53 guys like him."
"We made a decent investment in him because we thought he'd be pretty good. But he's everything and more you could have hoped for, I will say that. His personality, his intelligence, his toughness, his character both on and off the field, his leadership is all exemplary. You couldn't find anything better."
ESPNBoston.com's Lee Schechter and The Associated Press contributed to this report.