Ashlee Simpson and Evan Ross are set to release their debut single 'I Do' on Friday (07.09.18).

Evan Ross and Ashlee Simpson

Evan Ross and Ashlee Simpson

The couple - who tied the knot in 2014 and have daughter Jagger, three, together - are set to make their debut as a musical pair later this week, when they premiere their first single on Ryan Seacrest's iHeartRadio show, and later make it available for download on Friday.

'I Do' not only marks the first single from Ashlee and Evan as a couple, but it's also the first new music from singer Ashlee, 33, in over a decade.

The pop ballad was written by Ashlee and 30-year-old Evan - who is the son of music icon Diana Ross - alongside legendary bassist Verdine White of Earth, Wind & Fire, Austin Brown, Brian London, and Palmer Reed.

Speaking in a press release, the couple said: "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. 'I Do' is our commitment to our music and each other."

'I Do' will be featured during the first episode of the pair's E! reality show 'Ashlee + Evan', which is set to take fans behind-the-scenes on their journey through love, making music and raising a family.

The show will premiere in the US on September 9, and air in 160 across the world.

Meanwhile, the couple recently celebrated their fourth wedding anniversary on social media.

Posting a picture of the pair as they headed out to dinner, Ashlee - who also has son Brox, nine, with former husband Pete Wentz - wrote on Twitter: "Happy anniversary my love!! You make everything so fun, and beautiful!! I'm thankful for the day you walked into my life. I love you with all my heart. (sic)"