[1] Delfino, named for the dolphin, was laid down on 27 October 1928 at the Cantieri Riuniti dell'Adriatico (CRDA) shipyard at Monfalcone.

In December 1936 she patrolled the eastern coast of Spain during Spanish Civil War, making one failed attack against a merchant ship.

On 15 August 1940 Delfino it was involved in a disgraceful incident during which it torpedoed and sank the neutral, at the time, Elli, which was at anchor off Tinos, attending the Virgin Mary celebrations as honour guard.

On 29 November 1940, in the northern Aegean, Delfino fired two torpedoes at a convoy, and reported that the Psara had been hit, but this was never confirmed.

On 1 August 1941, on patrol off Tobruk, Delfino was attacked by a British Short Sunderland aircraft of 230 Squadron, and succeeded in shooting it down, and rescuing four men from the crew of twelve.

[4] Following further patrols around Sicily and Malta, in February 1942, Delfino was assigned to the Submarine School at Pula for training missions.

She returned to active duty in November 1942, based at Taranto, carrying out three transport missions to North Africa, taking over 200 tons of ammunition and fuel.