Having launched his professional career at Pontypridd, Jenkins switched to the Celtic Warriors before joining the Blues in 2004. He won his 100th Test cap at the start of the RBS 6 Nations campaign against Ireland and is hoping to add a third Lions tour to his three Grand Slams.
A delighted Cardiff Blues director of rugby, Phil Davies, said of his latest recruit: ""Gethin is an international player, a Lion and a quality player - I'm delighted that he has come back, it is a fantastic signing for us.
"It is great to be able to bring a senior Wales player back. We started the year knowing we had a young squad and it has been a massive season of progression, 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 32-year-old Jenkins will win his 97th Welsh cap in Rome on Saturday when Wales take on Italy at Stadio Olimpico. He played in five successive Lions tests in 2005 and 2009 before injury ruled him out of the third Test in Johannesburg four years ago.