The pair are known to millions as Nessa Jenkins and Bryn West - but this new show will see them appear as two brand new characters.
They'll play the title roles in a new half-hour comedy called Ray and Gaynor, the Daily Mirror newspaper reports.
Filming in South Wales, the one-off comedy is set to air as part of BBC One's festive schedule.
An insider told the newspaper: "It's a festive half-hour comedy for this year starring Ruth and Rob, who are old friends, in the title roles as Ray and Gaynor.
"Everyone is delighted to have a little bit of Gavin & Stacey on the BBC this Christmas. Even if it doesn't have Gavin. Or Stacey.”
The news comes after Ruth wouldn't rule out more episodes of Gavin & Stacey.
The show last aired a Christmas special in 2019 which peaked with more than 12 million viewers.
Ending on a cliffhanger as Ruth's Nessa character proposed to Smithy, played by James Corden, fans have been wanting another instalment ever since.
Ruth told The Sun newspaper: "I do say never say never, as while we did make it work that was after three years of trying to find time when we could sit down and write it.
"Obviously with the way it ends, there is room for more. It’s a shame we didn’t have more time when we were writing so we could have written more, then have it all ready to go."
For now you can watch Gavin & Stacey online via the BBC iPlayer here.