Although financial terms of the deal were not disclosed by the club, the NFL Network reported it was worth just under $20 million, making Boswell among the league's top-paid kickers.
Boswell earned his first Pro Bowl selection in 2017 by converting 35 of 38 field goal attempts, including all four chances from beyond 50 yards.
Entering his fourth season with the Steelers, Boswell came through repeatedly in the clutch by nailing four game-winning field goals last year -- three of them as time expired.
The 27-year-old Boswell tied an NFL record dating to the 1970 merger for the most game-winning kicks as time ran out on the clock. That included a 53-yarder to beat the Green Bay Packers, which tied for the longest field goal at Heinz Field.
"You've got to remember the same thing you do every time," said Boswell, per Pittsburgh's official team website. "You can't put too much pressure on one kick, not enough on another kick. It's just kick by kick. You just move on with it."
Boswell's 35 field goals established a franchise record and marked the first time a Steelers kicker converted at least 30 attempts since Shaun Suisham in 2013. Not only was Boswell's 92.1 percent success rate a career high, but he also was 37 of 39 on extra points to finish with 142 points -- also a Pittsburgh single-season record.
"Nothing is too big for this guy," said special teams coordinator Danny Smith. "He executed like he was supposed to and deserves everything he gets. I love the guy. He is a clutch guy. I would want him in any situation."
Boswell has appeared in 43 games over three seasons with the Steelers, converting on 85 of 95 field goal attempts and 99 of 102 PATs.