"My baby love and I are ENGAGED!!!" Ashlee, 29, told fans. "Hallelujah Hawaii!! The singer also shared a photograph of herself in her husband-to-be's arms, her huge new engagement ring proudly on display.
Evan, 25, posted the same picture, tweeting, "The love of my love said YES!!!!!"
Ashlee, the younger sister of Jessica Simpson, and Evan went public with their romance in July. The following month, the singer was spotted getting to know her future mother-in-law Diana Ross at a concert after party.
Speaking at the Hunger Games premiere in November, Evan hinted that marriage was on the cards, describing Ashlee as "the one".
"I'm so in love and we've got amazing things going on," he told the Insider. "And we inspire each other."
This will be the second time that Ashlee has tied the knot. She was previously married to Fall Out Boy rocker Pete Wentz, with who she has a five-year-old son Bronx.