The 56-year-old actress confirmed the news in an Instagram post Thursday.
Ryan shared a drawing of herself and Mellencamp holding hands, writing, "ENGAGED!" Fans congratulated the couple in the comments.
"Wonderful news congratulations," one person wrote.
"Sending blessings to you both!" another added.
Ryan had sparked engagement rumors in October. The star was spotted wearing a diamond ring on her left hand during an outing in New York, according to Entertainment Tonight.
"She hates me to death," the star told host Howard Stern. "She just doesn't want anything to do with me, and I don't blame her."
"I think it's because I'm a child," he said. "I throw fits, I gripe, I complain. I'm moody. Every bad thing that a fella can be, that's me."
Ryan was previously married to Dennis Quaid, while Mellencamp was wed to Priscilla Esterline, Victoria Granucci and Elaine Irwin. The pair both have children from previous relationships.