The news of their engagement comes nearly seven months after the arrival of their first child, a baby girl, whom they named Banks Violet Blair.
Duff and Koma met in 2015 while working together on the former's album Breathe In. Breathe Out. She was first linked to Koma, a singer/songwriter, in early 2017 when they made their red carpet debut at a SAG Awards party.
Duff previously dated fitness trainer Jason Walsh in late 2016, but split with him before the end of the year.
Duff started dating Koma in early 2017. They briefly broke up in March, though reconnected in September 2017.
In June 2018, Duff announced on Instagram that she and Koma were expecting a baby girl — her second child after her son Luca, whom she had in March 2012 with now ex-husband Mike Comrie.