The 24-year-old center will earn $2.6 million next season, $3.3 million in 2018-19 and $3.4 million in 2019-20.
Pageau had eight playoff goals last spring as the Senators reached the Eastern Conference finals. He had 12 goals and 21 assists in 82 regular-season games.
"As an organization, we are thrilled that we got this contract done," Senators general manager Pierre Dorion said.
"Arbitration is definitely a path we did not want to go down with Jean-Gabriel, especially the way he's performed over the course of the last few seasons. He showed that he was definitely deserving of this contract.
"Any player that has a history of raising his game, whether it was in junior playoffs or [AHL] playoffs or NHL playoffs, I think you have to give credit for what he's accomplished. We're really happy to have him on board for the next three years."
In 251 regular-season games, Pageau has 101 points (45 goals, 56 assists) and 71 penalty minutes.
Pageau had four goals -- including the game winner -- in Ottawa's 6-5 double-overtime win over the New York Rangers in Game 2 of their conference semifinal. The Ottawa native, who grew up in nearby Gatineau, Quebec, has 16 points (12 goals, four assists) in 35 playoff games.
"We've always had a good relationship with Pierre, with [former GM] Bryan [Murray] before, with the team," Pageau said.
"They always knew that I wanted to stay with the team. I take pride [in playing] for the Senators. So obviously, I think that helped get the deal done. I'm happy ... to be here for the next three years."
Pageau was drafted by the Senators in the fourth round (96th overall) of the 2011 draft.