The device is thought to date back 80 years to the Spanish Civil War.
The incendiary device measures 1.10 meters in length and and 80 centimeters in diameter and is reported to date from a 1936 bombardment ont he Catalan capital launched by the Italian air force on the orders of Mussolini who, like Hitler's Luftwaffe, came to the aid of Franco during the conflict.
Police established a security perimeter of 250 meters around the device off the Sant Sebastia beach, one of Barcelona's most popular beaches.
Navy specialists began the operation to deactivate the device on Monday morning.
Officials said the closure of the area would remain in place until it is made safe.
The device, located at a depth of three metres, 25 metres from the shore, was found by an off-duty police diver who happened to be swimming in the area.
The bomb contained 70 kilogrammes (150 pounds) of trinitrotoluene, a “highly volatile” substance, forcing explosives experts to move it one nautical mile away at sea before detonating it, a spokeswoman for the Civil Guard police force said.
Using diving lifting bags, designed to move objects underwater, they eventually detonated the bomb at a depth of 45 metres (150 feet), she added.