The Villans only narrowly avoided relegation last season, resulting in manager Dean Smith targeting the recruitment of a starting centre-forward for the 2020/21 campaign.
They have now landed Watkins, who was the top scorer in the Championship last season with 26 goals.
Watkins made the playoff final at the end of last season only for Brentford to be defeated by Fulham, but he has earned a place in the Premier League regardless with the transfer to Villa, which could rise to £33m.
"Ollie has developed into one of the most sought-after strikers in the country," said Smith
"We are looking forward to seeing him show his great qualities in an Aston Villa shirt.
"He is a hugely determined character who has both the ability and personality to succeed at the highest level."
The Villa forward line scored just six goals between them last season, including five from Wesley, who was a club-record signing last summer after joining for £22m.
Watkins is Smith's second recruit of the summer after having landed Matty Cash from Nottingham Forest for £16m.
Villa had previously been interested in Callum Wilson after Bournemouth's relegation, but Newcastle won the race to sign the Cherries striker.