Davies, the former Leeds and Scarlets boss, has left his role as the Welsh Rugby Union's academy manager with immediate effect to join Richard Hill's coaching team at Sixways.
"It has been a very difficult decision to leave the WRU but I have a huge passion for coaching and hope one day to coach Wales," said Davies. "With this in mind, I'm greatly appreciative of Worcester Warriors who have given me the opportunity to return to professional coaching, and to the WRU for considering and granting my request to do so."
During his decade in charge at Leeds, Davies took the club from the fourth tier of English rugby into the Premiership, winning the Powergen Cup in 2005 and qualifying for the Heineken Cup.
Davies guided the Scarlets into the Heineken Cup semi-finals before moving to the WRU, where he combined coaching the under-20s with helping Cardiff win the 2009 Principality Premiership title.
In May last year, Davies was appointed manager of Wales' new national academy.
Joe Lydon, the WRU's head of performance and development, said: "Phil is a highly experienced and accomplished coach and I am sure he will make a great success of his new venture.
"He leaves with all our best wishes and sincere gratitude for the excellent contribution he has made, both operationally and strategically within the WRU as national academy manager and as head coach to the Wales Under-20 squad during the last two seasons.
"I remain confident that the structures and systems Phil has helped put in place will benefit the game in Wales for years to come."