The 19-year-old, who rejected a new contract at the Nou Camp, has joined the Potters squad for their matches in the Asia Trophy in Singapore.
El Ouriachi, who links up with ex-Barca players Bojan and Marc Muniesa, played for Spain at youth level but has also been selected by Morocco Under-23s.
“Moha’s excited about joining a Premier League club,” said Stoke chief executive Tony Scholes.
“He has seen the progress made by Bojan and Marc and wants to try to do likewise.”
Stoke have not revealed the fee they have paid for El Ouriachi, or his contract length.
The Potters, who finished ninth in the Premier League last season, have now made seven summer signings.
El Ouriachi joins striker Joselu, goalkeepers Shay Given and Jakob Haugaard, defenders Glen Johnson and Philipp Wollscheid plus midfielder Marco van Ginkel in joining the club.