The 24-year-old Englishman moves to Villa Park on a five-year contract.
Watkins was joint top scorer in the championship last season alongside Fulham’s Aleksandar Mitrovic, as his 26 goals helped the Bees into the play-off final before losing to Scott Parker’s side.
Villa tried to sign Callum Wilson from Bournemouth earlier this week but were beaten to his signature by Newcastle United.
Villa manager Dean Smith reunites with Watkins, who he signed from Exeter City during his time as Brentford manager three years ago.
"Ollie has developed into one of the most sought-after strikers in the country and we are looking forward to seeing him show his great qualities in an Aston Villa shirt," Smith said.
"He is a hugely determined character who has both the ability and personality to succeed at the highest level."
Watkins added: "I am so excited to join Aston Villa and to reunite with Dean Smith as my head coach. I cannot wait to wear the famous claret and blue in the Premier League. I want to do something special here and score some goals."
Watkins’ transfer fee beats the £22m Villa paid to sign Brazilian striker Wesley from Club Brugge last summer.
Aston Villa scored 41 goals in the Premier League last season but only six of those came from strikers and manager Smith will be hoping the addition of Watkins will help prevent a repetition of a relegation battle.
