The striker has signed on a five-year deal which will keep him at the club until 2021. The buy-out clause in Alcácer's contract is set at 100 million euros.
After joining the La Liga giants, Alcacer spoke to Valencia's official website where he said: "When I found out about the offer from FC Barcelona I asked the club to listen to it, as it was my decision to take a step forward in my career.
"I want to thank Valencia CF and the fans. Despite the fact that I am leaving Valencia CF, I will always be a Valencianista at heart."
Paco Alcacer becomes FC Barcelona's sixth signing of this season after defenders Samuel Umtiti and Lucas Digne, midfielders André Gomes and Denis Suárez and goalkeeper Jasper Cillessen.
In another news, Barcelona have sent Moroccon international winger/striker Munir El Haddadi on loan to Valencia. Valencia have an option of converting the loan deal into a permanent one later.
Alcacer is a talented young forward who had grabbed many eye-balls with his impressive performance for Valencia last season. He scored 15 goals for the club in all competitions in 2015/16.
The Spaniard provides a perfect backup option for Luis Suarez to manager Luis Enrique.