The 31-year-old, who previously had playing spells at Richmond and Llanelli, is widely regarded as one of the best lineout takers in the top-flight and won England A honours in 2000.
Director of Rugby Mike Ruddock revealed he was delighted to have secured the signature of Gillies amid strong interest in the lock from other Guinness Premiership outfits.
"I'm delighted that Craig has signed for the next three years," he said. "He has been an outstanding player during his time at Warriors and his great form in the recent win over Bristol showed just how devastating he can be on his day.
"Overall I feel this season he has added a new dimension to his game and his tries against the Ospreys and Montauban at home showed the progression in his game.
"Given Craig's excellent attitude towards his physical development and conditioning, I see no reason for his form to diminish over the next three years."