The singer – who fronted the chart-topping band for more than three decades – is leaving the group to focus on his solo career and his acting pursuits.
He told Music Week: “I will be spending more time on my solo work – performing concerts, acting and my own songwriting – as an artist I feel a lot more settled in this world.
“I have had a great time and loved my career with Wet Wet Wet and to me they will always be the best band in the world.
“When I started in Wet Wet Wet I gave it 100% of my heart and soul and that’s what it demands and that is also what the fans demand – and if I can’t do that because my focus is elsewhere, then this is not fair on the fans or the rest of the guys in the band.”
The Scottish group now comprise of Tommy Cunningham, Graeme Clark and Neil Mitchell.
Wet Wet Wet’s last performance as a foursome happened recently at Edinburgh Castle.
The group performed three sold out shows to celebrate the band’s 30th anniversary.
Marti will now focus on his solo career as well as his acting pursuits.
He is currently co-writing a new stage musical with long-term collaborating Grant Mitchell and playwright Jack Bradley.
The band are best known for their chart topping song Love is All Around, a cover of the Troggs song, which is the best-selling ballad in British music history, selling 1.8million copies since its release in 1994.
It was only dethroned from the charts after 16 weeks when Whigfield’s Satrurday Night was released.