كتب : Ahmad Saied
The former La Liga champions said on their website that the deal was finalized after Joaquin passed a medical but didn't reveal any financial details.
Spanish sports daily Marca, though, said the move was worth 25 million Euros.
The 25-year-old winger was emotional while officially introduced as Valencia player.
"I want to thank the fans a lot, I will always love Betis," Marca quoted a weeping Joaquin as saying in the press conference.
"I have been devoted to the club during the six wonderful years I have been playing there and fans treated my like an idol.
"But now I felt that my role with Betis has finished and I am honored that Valencia have interest in me."
Joaquin is the second big signing to arrive to Valencia this summer after the Mediterranean club landed Spanish international striker Fernando Morientes from Liverpool.