Boychuk received a seven-year deal Thursday, as Nick Leddy did late last month. Boychuk's contract is worth $42 million, slightly higher than Leddy's deal for $38.5 million.
Both players were acquired by the Islanders in trades Oct. 4, just before this season. Boychuk came from Boston, and Leddy from Chicago.
This deal keeps the 31-year-old Boychuk from becoming an unrestricted free agent this summer and under contract through the 2021-22 season. Leddy, 23, could have been a restricted free agent.
Boychuk's salary-cap charge will rise from this season's $3.367 million to $6 million in each year of the deal.
Boychuk was acquired from Boston for a second-round pick in this year's draft and a second-round choice in next year's draft.
"The organization has been incredible to both myself and my family," Boychuk said in a statement. "I am proud to be an Islander and look forward to competing every night in front of such a passionate fan base for many years to come."
Boychuk leads Islanders defencemen with 32 points, 25 assists and a plus-17 rating in 59 games. He has established career highs in goals, assists and points in his ninth season in the NHL.
He will earn $7.5 million next season, $7 million in each of the following two seasons, $6.5 million in the 2018-19 campaign, $6 million the next season, and $4 million in both of the final two years of the deal for a $42 million total.
"Johnny's influence in our dressing room, both on and off the ice, has been immeasurable," Islanders general manager Garth Snow said in a statement announcing the deal. "His veteran presence is an asset that we are thrilled to help lead our club."
In 380 NHL games with Colorado, Boston and the Islanders, Boychuk has 26 goals and 81 assists. In 79 playoff games, including a Stanley Cup title with Boston in 2011, he has 13 goals and 14 assists.
He joyfully announced his new deal on Twitter, invoking the popular chant fans have used to celebrate every home goal this season in the final year at Nassau Coliseum.
"'Yes! Yes! Yes!," Boychuk wrote, "For seven more years!!"
Leddy had played in New York's first 68 games this season before an injury Monday forced him to miss the Islanders' home matchup with the Rangers the following night. That ended his consecutive game streak at 320.