The Portuguese striker, 23, moves on loan to the Camp Nou after a spell at Chelsea during the second half of last season.
Chelsea were briefly linked with Felix in recent days but Barcelona pressed ahead with their deal to sign him from Atletico Madrid.
Atleti had been hoping for the player’s salary to be covered in full along with a £6.5million fee for his temporary exit, after Felix and coach Diego Simone failed to reconcile their public falling out from earlier in the year.
The figures of the deal have not been confirmed but Barcelona had been struggling to finalise their latest signings due to ongoing financial issues.
Despite departing Madrid, Felix has put pen to paper on a new contract with Los Rojiblancos. A two-year extension takes his deal up to 2029.
Barcelona have also signed Joao Cancelo on loan from Manchester City.
Earlier on deadline day, Clement Lenglet departed for Aston Villa while Ansu Fati’s loan move to Brighton has also been confirmed.