Watford has signed Gerard Deulofeu from Barcelona for a fee of £11.5 million ($15.4 million).
Deulofeu spent last season on loan with Watford.
The deal with Barcelona also includes a sell-on clause, according to the Spanish club.
"We are very pleased to welcome Gerard back," said Watford chairman Scott Duxbury. "Our ambition to succeed and grow in the Premier League remains as strong as ever."