The 32-year-old left the Blues last summer to take up a two-year contract with the big-spending French Top 14 outfit.
But the two-time British & Irish Lion, who has won 96 caps for Wales, has had to play second fiddle to Andrew Sheridan and had been linked with a rapid return to his former side earlier in the season.
The move bucks the trend of Wales players moving across the Channel, with centre Jamie Roberts and flanker Dan Lydiate set to be next to make the switch.
Cardiff Blues director of rugby Phil Davies said: "I am really pleased to have re-signed Gethin.
"We started the year knowing we had a young squad and it has been a massive season of progression, (although) sometimes the results have not gone our way.
"Gethin will be coming back to an environment that is a little different from the one he left and I think, hopefully, he will enjoy it and I know that he will add a massive amount to the group.
"I have heard how professional he is and I am really looking forward to working with him and welcoming him back to the Blues.
"Gethin's experience will add to the team, he is very vocal on the pitch and I am sure that a lot of the youngsters will also be very vocal with him as well next year.
"The experience of Gethin and Matthew Rees, along with the likes of Gavin Evans and Dafydd Hewitt who are really experienced players, will help."