Witnesses could not positively identify the species that attacked the boy but some described it as a blue shark (Prionace glauca), a wide-ranging species common in the Mediterranean Sea.
Salou town hall expressed its regret over the incident which it described as "isolated" according to Catalan daily La Vanguardia.
The boy was in the area on holiday with his family from the northerly Spanish region of Aragon.
He was reported to have been playing with an inflatable mattress in shallow water when he was bitten on the right hand.
Fast-moving, highly migratory blue sharks usually eat fish although they are known to sometimes go after larger prey.
The sharks, which can weigh up to 80kg (175 pounds), can become aggressive if they feel threatened but rarely attack humans. As of 2013, blue sharks had only ever been implicated in 13 attacks upon humans, four of which ended fatally.
A number of Catalan beaches were temporarily closed earlier in the summer when 3 blue sharks were spotted in the water.