The 27-year-old second-row - who is 6ft 5in tall and weighs 18st 8lb - joined Northampton Saints in 2005 before later joining Harlequins in the summer of 2007.
Percival, who previously studied at Worcester Sixth Form College and Worcester University, has made a total of 57 appearances for Quins and will now join up with Warriors on a two-year deal from the start of the 2011/12 campaign.
"Phil Davies and I have been looking closely at the make-up of our pack of forwards for next season and we firmly believe that James Percival will be a fantastic acquisition for Warriors," said Head Coach Richard Hill.
"James offers huge physicality, will do exactly what we want him to do on the field and adds a real abrasive and hard edge to our pack.
"He also brings with him the added benefit of having been here before so he knows the club and area very well and will settle in quickly.
"Everyone I have talked to at Worcester who has worked with James in the past has spoken very highly of him and the same can be said of Harlequins who I know will be sad to see him leave.
"It's fantastic to get a player back who clearly cares so much for the club and we hope in the future that other players who have left us will be able to make a return and big impact."