"Ryan has all the qualities to be successful in the Premiership," said Worcester director of rugby John Brain.
"He will add to the depth of resources we have at scrum-half and greatly increase the amount of competition for a starting spot."
Powell will compete with former England scrum-half Andy Gomarsall for a place in the Warriors line-up.
"These are exciting times for me as a player," said Powell.
"It's a great opportunity and I am looking forward to the challenge of playing in the Guinness Premiership and coming up against some of the best scrum halves around every week.
"My main priority is to win the confidence of the players and coaches and to secure a starting spot."
Worcester still have the chance of Heienken Cup qualificiation next season. Their 34-25 European Challenge Cup quarter-final win over Northampton last weekend means they face Gloucester in the semi-final. The winner of the tournament is granted an anutomatic place in the Heineken Cup, and with Worcester in mid-table in the Guinness Premierhip, it represents a 'golden ticket' to European rugby's top table.