29-year-old Simpson got engaged during their Hawaii trip recently.
"My baby love and I are engaged! Hallelujah Hawaii!," she posted on Twitter.
"The love of my life said yes!" Ross tweeted and also shared a photo of himself and Ashlee.
The couple had first sparked dating rumours in July after they were seen holding hands on a night out in Los Angeles.
This will be the second time the 'I Am Me' singer will tie the knot.
She was previously married to 'Fall Out Boy' bassist Pete Wentz from 2008 until 2011. They together have a five-year-old child.
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