The 30-year old, an Ospreys 'original' with 123 regional appearances to his credit from two prior spells in the black jersey, will report for duty at Llandarcy Academy of Sport this week.
Bangor born Griffiths originally signed for the region from Caernarfon RFC in the summer of 2003 ahead of the first ever season of regional rugby, and made his debut as a replacement in a 35-21 away defeat to Leinster in September of that year. His 100th appearance came away to the Dragons in April 2011.
A part of four league winning campaigns, he left the Ospreys for London Irish in the summer of 2012, returning on loan six months later as much needed injury cover after the region lost three front-line tight-heads. He then spent 2013/14 at London Welsh, helping them clinch promotion back to the Aviva Premiership at the first attempt.
Recruitment and Retention Manager, Andy Lloyd, said Cai will add much needed experience and cover in a key position for the coming season:
"It's positive news for us in that he is someone with bags of experience in a key position, who will be available to us all season long" said Lloyd.
"He knows, understands and values what it means to be an Osprey and what is expected of anyone who wears the jersey.
"On average we are using 10 to 12 props over the course of a season so it's a position where you can't have too many options. There's been a lot said about the young talent we have coming through in the front row but to give them the best chance of fulfilling their potential you have to provide the right support on and off the field. Cai's experience will certainly help with that, particularly when our international props are away for long periods.
"He's a durable character, a big presence around the environment, and moving away from the region over the last couple of years has allowed him to mature and realise exactly what he had here. In his loan period two seasons ago I think we really saw the best of him, it was the best he's played in an Ospreys shirt. With the experience gained from different environments in recent seasons, we expect him to come back into the group and be a positive influence."
Griffiths is the sixth player to sign for the Ospreys ahead of next season, following on from Worcester and Fiji centre Josh Matavesi, Eastern Province Kings second row Rynier Bernardo, Newport Gwent Dragons pair Dan Evans (full back) and Sam Parry (hooker), and Carmarthen Quins loose-head prop Gareth Thomas.
In addition 14 players have signed new deals since the start of 2014, including skipper Alun Wyn Jones, while flanker Lloyd Evans (Bridgend) and centre Aled Jenkins (Swansea) have been rewarded for their form in the Premiership with their first development contracts. Seven local youngsters have also been promoted into the B Group, signing development contracts as they progress along the regional development pathway from the Ospreys age-grade set-up.
"It's good to be back where I consider my home region" said North Walian Griffiths.
"I had an amazing time with London Welsh last season and was part of a special experience, but I'm really happy to be rejoining the Ospreys. I'm looking forward to a fresh start and another challenge.
"There are good people here, good coaches, a strong squad, a great support base and facilities that have changed beyond recognition since we started in 2003. The Liberty Stadium is an incredible venue which is special to me, and it's great to be back.
"I've played with and under Steve Tandy many, many times, and I know where he's coming from with what he wants from the players and demands from the team. That will help with the transition back into the Ospreys environment, but there are a lot of new faces around the place so I'm looking at it as the start of a new era.
"I'm not a youngster anymore, I'm a bit older and I've experienced different environments. I've grown as a player and a person since I first left here in 2012, and hopefully I can make an impact with the Ospreys in the coming season."
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