The 55-year-old actress and 65-year-old musician reconciled in the spring after ending their relationship in 2014, according to Page Six.
"They are together. It's been a few months," a source told the publication.
People confirmed the You've Got Mail star and the "Jack & Diane" singer have rekindled their romance.
"They have a bond," an insider explained.
"Oh, women hate me. I loved Meg Ryan," he told host Howard Stern. "She hates me to death."
"She just doesn't want anything to do with me, and I don't blame her," the star said. "I think it's because I'm a child. I throw fits, I gripe, I complain. I'm moody. Every bad thing that a fella can be, that's me."
Ryan and Mellencamp were spotted together in New York the same month, and stepped out to the Art Cliff Diner in Martha's Vineyard, Mass., in May. Page Six reported Ryan attended Mellencamp's concert July 11 in Forest Hills, N.Y.