Traore becomes Villa’s tenth signing of a busy summer at Villa Park as manager Tim Sherwood has sought to overhaul his squad in his first summer in charge.
The deal for the Spanish winger is worth an initial £7.1m but could rise to £8.6m if certain conditions are met.
Barca have also inserted a three-year buy-back clause in the 19-year-old’s contract at Villa Park.
Sherwood said: “He’s someone I have been tracking for a long time.
“He’s physically a man – and he has been for some time. He’s a big powerful boy, very quick, dynamic and a maverick-type player. Those players are few and far between.
“We’re delighted he has chosen Villa to continue his development.”
Traore, who joined Barca’s youth set-up in 2004 at the age of eight and played for them in the Champions League, is looking forward to developing under Sherwood.
He said: “It’s an honour for me to come here. It’s a club that showed interest in me from the first moment and made me feel important in terms of their project.
“I have spoken to the manager and have a very good impression of him and what he wants from me. All this have made this club my top attraction.
“I would also like to express my gratitude to everyone at FC Barcelona for helping me develop as a player and as a person.”