The Grammy-winning musician, whose other hit songs include 'Hurts So Good', has been romantically linked to Ryan since 2011.
E! News reports the pair have had a seven-year on-again, off-again relationship: they first broke up in 2014 before getting back together that same year, before splitting in 2015 and reconciling in 2017.
Ryan announced the news via a quirky illustration on her Instagram:
This will be 56-year-old Ryan's second marriage, after her first to Dennis Quaid, news.com.au reports. They have a child together, 26-year-old Jack, and Ryan also has an adopted daughter, Daisy.
Mellencamp, 67, has been married three times previously and has five children, including Real Housewives of Beverly Hills star Teddi Mellencamp Arroyave.