The 26-year-old, who can play as a number eight or flanker, has signed a three-year deal at the Madejski Stadium.
London Irish director of rugby Brian Smith told BBC Sport: "Blair Cowan is a player we've been tracking for some time.
"There are a lot of tough players in this league, but not a lot tougher than this guy.
"Blair is a great fit for us, he can play all three back row positions and he's a try scorer."
Cowan has made 20 appearances for Worcester, scoring seven tries.