The 85-year-old baseball lifer spelled out his allegations in a new lawsuit, obtained by TMZ Sports, alleging the age discrimination happened after he took over the interim pitching coach role in the middle of the 2019 season.

Regan, in the suit, says after he filled in for the fired Dave Eiland, the team saw immediate results. He claimed the staff's aces -- Jacob deGrom, Zack Wheeler and Noah Syndergaard -- all got better under his watch ... and he added that the team's ERA lowered significantly too.

But, he said he was not retained following the season ... claiming he was not brought back solely because he was 82 years old at the time.

In the suit, filed by high-profile employment attorney Matthew Blit, Regan said the team's GM at the time, Brodie Van Wagenen, even admitted to him that he wasn't coming back because he was "too old."

Regan also said in the lawsuit that Van Wagenen told him, "If we don't find the young pitching coach we are looking for, we will hire you back."

The Mets ended up hiring 33-year-old former MLB pitcher Jeremy Hefner to replace Regan in the offseason.

Regan -- who both played and coached in the MLB for six decades -- did not coach again following the '19 season. He's suing for unspecified damages.

"Phil is the first to crack jokes about his age," Blit said in a statement to TMZ Sports, "however there is nothing funny about a person being told that they are being passed over because their employer wants someone younger. That's discrimination, plain and simple."

Blit added, "Suing for discrimination in professional sports is extremely rare. In fact, Phil debated doing this for many years. But the hurt from the conversation that he had with Mr. Van Wagenen continues to haunt him to this day. He can hear the exact words playing in his head over and over. Phil knew this had to be done."