The track, released Thursday on YouTube as a lyric video, is expected to appear on the couple's planned duets album that is set to arrive this fall.
"Whenever I lose my way, you help me / This love came over me like a hurricane / But I know you got me / Out the darkness, I can see / And baby, there's no place I'd rather be," Simpson sings.
"I Do" is Simpson's first song since 2012 following the release of "Bat for a Heart." The recording of the song will be featured on the pair's upcoming E! reality series Ashlee + Evan, which premieres on Sept. 9.
"Music was always one of the things that connected us from the beginning. We liked our voices together, and as time went on, we thought it was something that we wanted to share," Simpson and Ross said in a statement. 'I Do' is our commitment to our music and each other."
"I Do" is also available on music streaming services such as Spotify, Apple Music and Tidal.