"Engaged", Ryan wrote in the caption of the quirky announcement post, featuring a hand-drawn cartoon of her and Mellencamp holding hands.
Mellencamp has been married three times before, most recently to fashion model Elaine Irwin, whom he divorced in 2011 after 18 years of marriage.
The couple started dating in 2011 but split in 2014, with the musician telling satellite radio shock jock Howard Stern, "Oh, women hate me".
The outing comes just a day after she announced her pending nuptials to the 65-year-old rocker she has dated on and off for seven years.
The Sleepless in Seattle actress and the Jack and Diane singer-songwriter rekindled their relationship in 2017.
Congrats to the happy couple on the news! "She hates me to death". I think it's because I'm a child. I'm moody. Every bad thing that a fella can be, that's me, ' he told Stern.
Talk of an engagement began after Ryan was spotted wearing a new sparkler on her ring finger on Wednesday in NYC (inset above). Mellencamp has been married three times and has five children of his own. Even "Real Housewives of Beverly Hills" star and Mellencamp's daughter, Teddi, headed to her Instagram page to express her happiness over the change in their relationship status.