He becomes the 13th new face at Warriors this season ahead of a return to the top-flight of English rugby and follows Ben Gulliver in making the move from Pirates.
The Bristol-born back row, who is 6ft 2in and 15stone, was regarded as one of the finest prospects at Pirates having made a total of 29 appearances last season.
The 24-year-old joined the Pirates in 2005, having previously played at Truro, and made his debut in the same year against Earth Titans. He was named the club's Most Improved Player at the end of the 2005/06 campaign.
Betty, who has represented Cornwall at Under-20 level, continued his eye-catching development by starting 24 of the 30 matches he appeared in during the 2009/10 season and made his 100th appearance for Pirates in February 2010 against Nottingham at Camborne.
He played his 136th and final game against Warriors in the second leg of the RFU Championship final in May.
"Sam is a player that I have been aware of for a number of years," revealed Head Coach Richard Hill. "Don Rutherford, the former Technical Director of the RFU, has kept onto me about this player, who has impressed year after year.
"He played particularly well against us when he came on in the second leg of the RFU Championship final. When Sam came on the Cornish Pirates looked dangerous and played very well in that final half an hour.
"Sam can operate at openside or blindside but we seem him primarily as a six. He is an aggressive ball carrier and destructive tackler. That aggression is an attractive proposition to us and he is hugely motivated with a big work rate."
Hill added that Betty would come into the squad with fellow back row Pat Sanderson set for a longer spell on the side-lines. Sanderson has been out of action since injuring his shoulder in the RFU Championship play-off clash with Bristol in March.
"Pat has taken longer than he and we would have liked to repair and remains in rehabilitation with us," said Hill.
"At the moment he is unlikely to make any of the pre-season games and therefore there is a danger that he will not be match fit for the start of the season.
"There is a need to ensure that we have six fit back row forwards going in the start of the Aviva Premiership. With that in mind, we have signed a very talented English back row in Sam Betty and we look forward to him joining up with the squad."