Al Pacino reportedly regrets not marrying Diane Keaton after their on-again, off-again relationship that spanned nearly two decades.

Keaton — who died Saturday in California at age 79 — played Pacino’s girlfriend in “The Godfather” trilogy, and they started dating in 1974. They called it quits in 1990, after Keaton gave him an ultimatum and he refused to commit to marriage.

Pacino, 85, now wishes he had tied the knot with the late actress, a source told the Daily Mail in an article published Sunday.

“I know he will forever regret he didn’t make his move when he had the chance,” a friend of the legendary actor told the outlet. “For years after he and Diane split, Al used to say, ‘If it’s meant to be, it’s never too late for a do-over.’ But sadly, now it is.”

“Looking back, Al admits the love of his life was Diane, who he’s always called, ‘an amazing woman,’” the friend added.

The source also said that Keaton and Pacino never spoke — despite having both lived near one another in Beverly Hills.

“I once asked him why, and he told me, ‘There’s no need to talk with each other. We said everything that needed to be said at the time,’” the source said.

A rep for Pacino didn’t immediately respond to Page Six’s request for comment.

Keaton looked back on her relationship with Pacino in a 2017 interview with the Sunday Times, saying that in hindsight, it was better the two didn’t ever wed.

“I didn’t even want him to propose. I just thought maybe he would marry me, eventually,” she recalled.

“I thought, ‘Don’t even propose — let’s just do it,’” she continued. “But that never happened, and that is a blessing for both of us. It would have been a nightmare for him.”

The Oscar-winning actress told the outlet that ultimately, they weren’t a good match.

“We’re very eccentric, he needed a woman that was going to take care of him, I needed a man who would take care of me … It was just very important that we left each other alone, said goodbye,” she shared. “But it wasn’t my choice.”

Keaton previously admitted that she was head over heels for Pacino after developing a crush on him while filming 1972’s “The Godfather.”

“I was mad for him. Charming, hilarious, a nonstop talker,” Keaton told People about her ex in 2017. “There was an aspect of him that was like a lost orphan, like this kind of crazy idiot savant. And oh, gorgeous!”

Keaton never married, but she adopted two children in her 50s — daughter Dexter, 29, and son Duke, 25. Pacino has also never been married. He has four children: Julie Marie, 35, twins Anton and Olivia, 24, and son Roman, 2.

Pacino made a rare public appearance at the 45th AFI Life Achievement Award Tribute to honor Keaton in 2017.

“You’re an artist, Di,” he told her during his speech. “You’re a great artist. … I love you, forever.”

The “Scent of a Woman” actor isn’t Keaton’s only ex reportedly struggling to cope with her passing.

Woody Allen — whom Keaton dated for a few years in the early ’70s and worked with on multiple films — is “extremely distraught” over Keaton’s death.