The deal is worth $55 million total, with a $6.875 million average salary that takes effect a year from now, sources told ESPN.com's Pierre LeBrun.
Seabrook, 30, was set to become an unrestricted free agent after the season. His last contract was for five years and $29 million. His new contract will run through the 2023-24 season.
Seabrook was drafted by the Blackhawks in the first round in 2003. He has played 763 regular-season games and 112 playoff games over 10 seasons with the Blackhawks. He has helped the Blackhawks to three Stanley Cup championships during his career, and in 2015 set the single-postseason franchise record among defensemen with seven goals.
Seabrook is considered one of the team's leaders and was recently designated an alternate captain.
The Blackhawks have two of their top defensemen locked up for many years to come. In addition to Seabrook, defenseman Duncan Keith is signed through the 2022-23 season.