The Mets trumpeted the start of their offseason Sunday by locking up one of their own.

Just four days before free agency officially begins — and all 30 teams could have taken a run at Edwin Diaz — the Mets agreed to bring back their closer on a five-year, $102 million deal, The Post’s Jon Heyman and Joel Sherman confirmed Sunday. The deal includes a $12 million signing bonus, a sixth-year option and an opt-out after three years, plus a full no-trade clause through the opt-out year.

The contract is a record for a reliever, surpassing the five-year, $86 million deal Aroldis Chapman signed with the Yankees in 2016. The average annual value of $20.4 million also exceeds Liam Hendriks’ reliever record of $18 million, set in his 2021 deal with the White Sox.

Diaz is coming off arguably the most dominant season of any reliever, in which he recorded a 1.31 ERA and 118 strikeouts across 62 innings. He had 32 saves in 35 chances, with no blown saves over the final four months of the season. In the process, he also became a fan favorite with his popular entrance music, “Narco,” which led to a live performance from Timmy Trumpet at Citi Field for one of Diaz’s appearances in August.

The hard-throwing right-hander, who pairs an electric fastball with a nasty slider, will turn 29 in March. He was a critical player for the Mets to bring back, especially because Drew Smith is the only other surefire reliever they have under contract for next season.

Diaz had been vocal about his interest in staying in New York, saying after the Mets were eliminated by the Padres in the NL wild-card round, “I hope the team comes quick to me to talk.”

The Mets followed through and made sure the 29 other clubs were never able to talk numbers with Diaz once free agency opens at 5 p.m. Thursday.

While Diaz is now committed to staying in New York, he did not arrive here by choice. He came over in a blockbuster 2018 trade from the Mariners with Robinson Cano for top prospect Jarred Kelenic, among others. Diaz then had a brutal introduction to Queens in 2019, posting a 5.59 ERA with 26 saves in 33 chances.

But his turnaround proved to be one of the better ones in recent New York sports history. He registered a 1.75 ERA during the COVID-shortened 2020 season, then proved it was not a fluke with a solid 2021 campaign, which he finished with a 3.45 ERA and 32 saves in 38 chances.

Diaz’s walk year with the Mets, though, was on a totally different level. He emerged as a shutdown weapon for manager Buck Showalter and helped the Mets by being open to pitching the eighth innings at times if the situation called for it. By allowing just 34 hits and 18 walks, his WHIP of 0.84 was the second-lowest of his career behind only 2018 with the Mariners (0.79), his only All-Star season prior to 2022.

Getting Diaz’s future wrapped up will allow the Mets to turn their attention to other areas this week as free agency begins. Their other in-house free agents include Jacob deGrom, Brandon Nimmo, Taijuan Walker, Chris Bassitt (who is declining a $19 million mutual option and will hit the open market), Adam Ottavino and Seth Lugo.

The Mets are expected to have plenty of competition for those players, but they made sure Diaz never got away. He officially became a free agent at 9 a.m. Sunday, but by the end of the day, he had agreed to a new deal with the Mets to ensure he is here to stay.