The 25-year-old son of Diana Ross tweeted a photo of himself and Simpson wearing her ring on Monday afternoon, writing, "The love of my life said YES!!!!!!"
Simpson, 29, tweeted the same photo.
Simpson has a 5-year-old son, Bronx Mowgli, from her previous marriage to Fall Out Boy rocker Pete Wentz. They divorced in 2011.
My baby love and I are ENGAGED!!! Hallelujah Hawaii !!!!! pic.twitter.com/KVwoltverE— Ashlee Simpson (@ashleesimpson) January 14, 2014