The 56-year-old actress was first romantically linked to Mellencamp, 67, in 2010.
She was bombarded with congratulatory messages as she shared a comical stick figure drawing of her and her other half on social media, alongside the simple caption: “ENGAGED!”
Ryan’s engagement news comes after she was pictured out and about flashing a diamond ring on that finger, fuelling rumours she was either about to tie the knot or had already walked down the aisle with Mellencamp.
This will be Ryan’s second marriage and the fourth for Mellencamp, 67. The When Harry Met Sally favourite shares a son, Jack, with her ex-husband, Dennis Quaid, and has an adopted daughter named Daisy. Mellencamp has five children from three past marriages.