The pop star couple, who married four months ago, announced their happy news online, with Rochelle writing:
"I wanted to be the first to tell you...we have some exciting news to share! Marvin and I could NOT be more thrilled to announce that we are expecting our first child mid next year.
"We have always dreamt of being parents and the news is the best early Christmas present we could have asked for. I know Marvin is going to be a great dad and we're so excited to start a family. Thanks for all of your support and lots of love, Rochelle x."
Marvin then took to Twitter to add: "Rochelle and I are SOOOOO excited to tell everyone that we are expecting our first baby!!! Its absolutely amazing!
"Baby is due middle of next year and all is great!! We want to say thank you to everyone for their love and support... it means so much to us!! We are over the moon! Love Marv x."
Rochelle will be the second mum in The Saturdays - fellow band member Una Healy welcomed baby Aoife Belle earlier this year.
Huge congratulations Rochelle and Marvin!