The Barcelona beaches of Masnou, Ocata (Masnou), Ponent, Descàrrega, Pla de l'Os and Bellamar (Premià de Mar) were shut down after Red Cross lifeguards saw a one and a half-metre (5-foot) long blue shark at Ocata beach near El Masnou.
Later searches revealed the presence of two other sharks.
A screenshot of the Catalonia Civil Protection website showing the location of the closed beaches
The animals were "very close" to shore in the area where people were swimming, a Red Cross spokesperson said on Tuesday.
The beaches were opened again on Wednesday morning, local daily LaVanguardia reported.
Blue sharks, which can weigh up to 80kg (175 pounds) can become aggressive if they feel threatened by humans.