The Senators made the announcement in the morning, hours before Ottawa's home opener against the Toronto Maple Leafs.
The 22-year-old Tkachuk was a restricted free agent. He missed the preseason and was not available to play Thursday night, with coach D.J. Smith saying earlier it would take Tkachuk at least a week to 10 days to play.
"This is an exciting day for the Ottawa Senators," general manager Pierre Dorion said. "Brady possesses a very dynamic skill set and is a prototypical power forward in today's NHL. He scores goals, he plays physical and exemplifies what it means to be a pro for all players in our organization."
The contract has an average annual value of $8.214 million. Tkachuk will be paid $4 million this season, $6.5 million in 2022-23, $10.5 million in 2023-24, $10.5 million in 2024-25, $10.5 million in 2025-26, $8.5 million in 2026-27 and $7 million in 2027-28.
"I'm just super happy to be here, super happy to be back in Ottawa,'' Tkachuk said. "It's kind of a crazy day and just words can't really express how happy and excited I am to be back.''
Tkachuk played in all 56 games last season and led the Senators in scoring with 36 points on 17 goals and 19 assists. The 6-foot-4, 211-pound left wing also had a team-high 69 penalty minutes and was second in the NHL in hits with 248.
"The one thing that always impressed me throughout this process is hearing from the other side about how much Brady wanted to be here long term," Dorion said. "That's why we were never worried that we weren't going to be able to do a long-term deal. And that says a lot about him.
"He's the one that drives the bus for our team. He brings us into battle every night. And to get it done for seven years, I think, is a great thing for the organization."
Selected fourth overall in 2018 after one season at Boston University, Tkachuk has 60 goals, 65 assists and 250 penalty minutes in 198 career NHL games. His father, Keith, starred in the NHL, and older brother Matthew is a forward for the Calgary Flames.