Cowan, who has made a total of 79 appearances for Pirates and scored 26 tries in league and cup action, moves to Sixways on a deal that will keep him at the Aviva Premiership side until at least 2013.
The New Zealand-born 25-year-old is expected to join up with his new team-mates at the start of next week.
Head coach Richard Hill said: "Blair is a very powerful man with a good rugby brain. We are after a strong ball carrying Number Eight who can make an impact for us and I know Blair is very keen to make a mark here.
"He has demonstrated during his time at Cornish Pirates that he is a real talent."
Cowan said: "I've watched the Warriors over the last few weeks and seen the fantastic results the club has achieved and the real team spirit that is clearly evident at Sixways. It's a great time to become a Warrior."
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