"Thomas is an exceptional talent and an outstanding teammate who is impactful both on and off the ice," general manager Pierre Dorion said. "He is the type of player that can develop into a core member of a championship-level team in the National Hockey League.
"He is an NHL All-Star; an elite skater and puck-mover who plays with pace and determination."
Chabot has one year remaining on his entry-level contract and was set to become a restricted free agent next summer. The 22-year-old's new deal will give him the highest average annual value on his team and make him one of the league's highest-paid defensemen.
The 6-foot-2 rearguard put together an impressive sophomore campaign in 2018-19 and was named to the Atlantic Division All-Star team. Chabot finished 10th in scoring among NHL defensemen, tallying a career-high 14 goals and 55 points in 70 games. He also led the Senators in average ice time with 24:17 per game and ranked third with 112 blocked shots.
Ottawa now has multiple key pieces locked up after inking forward Colin White to a six-year deal in August.
Chabot has recorded 23 goals and 80 points in 123 contests since the Senators selected him with the 18th overall pick of the 2015 NHL Draft. His 0.60 points-per-game mark leads all defensemen from that class.