PITTSBURGH -- After a contract negotiation that lasted throughout training camp, the Pittsburgh Steelers and outside linebacker T.J. Watt came to terms on a megadeal Thursday.

Watt, 26, agreed to a four-year, $112 million extension, a source told ESPN's Adam Schefter. The agreement includes $80 million in full guarantees, a source told ESPN's Jeremy Fowler.

The average annual value of $28 million per season makes Watt the NFL's highest-paid defensive player. Los Angeles Chargers defensive end Joey Bosa's five-year, $135 million extension signed in 2020 has a $27 million average and $102 million guaranteed.

Watt's deal also outpaces Chicago Bears outside linebacker Khalil Mack, who previously was the highest-paid linebacker with a six-year, $141 million contract signed in 2018. Mack's deal averages $23.5 million annually and has $90 million guaranteed.

With the deadline nearing, negotiations pushed into Week 1 as the two sides worked to find common ground on the contract's guarantees. The Steelers typically guarantee only the signing bonus, but not much else. Watt's case, though, broke that mold.

Watt isn't the first member of his family to make record-setting money. Older brother J.J. Watt, now with the Arizona Cardinals, signed a six-year, $100 million extension with the Houston Texans in 2014. J.J.'s $51.8 million guaranteed and $16.6 million average were both records for an NFL defensive player at the time.

T.J. Watt finished second to Aaron Donald in Defensive Player of the Year voting after his 15-sack campaign in 2020. He also added an interception and two forced fumbles to go with 53 total tackles. Watt has had double-digit sacks in each season beginning in 2018, recording 13, 14.5 and 15 sacks. His two forced fumbles in 2020 were his lowest since recording just one his rookie season. He had six in 2018 and eight -- along with four fumble recoveries -- in 2019. He finished third in Defensive Player of the Year voting to New England cornerback Stephon Gilmore and Arizona linebacker Chandler Jones in 2019.

"T.J. is visiting from another planet, to be quite honest with you," coach Mike Tomlin said during the 2020 season. "He has freakishly unique talent coupled with freakishly unique work habits and mindset, and it produces what you guys witness every week, which in my opinion is Defensive Player of the Year quality."

The Steelers parted ways with fellow outside linebacker Bud Dupree in free agency, signaling that they were moving forward with Watt as the cornerstone of the defense and planned to sign him to a lucrative deal. This year, he'll anchor the left side of the pass rush and be part of a rotation that includes former Chargers outside linebacker Melvin Ingram and second-year edge rusher Alex Highsmith.

The Steelers dealt with a tight cap in 2020 and 2021 in part because of quarterback Ben Roethlisberger's contract, but they restructured his deal and released longtime right guard David DeCastro before camp, freeing up space to sign veteran free agents and sign Watt to a megadeal.

After this season, though, the Steelers are projected to be in a much different cap situation with Roethlisberger's contract voiding after the season. That gave them the space to sign Watt to his deal.

Though he attended every practice, Watt didn't fully participate in training camp while the two sides negotiated the blockbuster payday. Instead, Watt worked individually with strength and conditioning coaches on the sideline at Heinz Field during team periods.

"Everybody knows T.J.," defensive coordinator Keith Butler said on Aug. 7. "Everybody knows that he's a good teammate. He wants to be out there. He really wants to be out there, but some things you've got to just do. This is a business. We all love to think it's a game, but when it's all said and done, it's a business."

Now that business is taken care of, Watt is back on the field this week and ready to go to work Week 1 against the Buffalo Bills.