She served during the Balkan Wars, earning the distinction of being the first submarine in the world to conduct offensive patrols and launch a submerged torpedo attack (albeit without success) against a warship.

Diving was performed with great caution to prevent an excessive negative inclination from the inflow of water in the ballast tanks.

[4] For surface running, the boat was powered by two Schneider-Carels diesel engines which were heated by compressed air for 10-15 minutes by their corresponding two electric motors before starting.

Construction of Delfin had begun in early 1909, before the Greek order was placed, based on the design of naval architect Maxime Laubeuf.

During the course of August and September the ship's crew underwent intense training under the guidance of French officers specializing in submersibles.

[8][9] Delfin sailed on 29 September, travelling on the surface and unescorted and arrived at Corfu on 4 October to repair damage to her left engine.

A day later she began daily conducting daylight offensive patrols (both submerged and on the surface) at a distance of 4,000–6,000 meters (13,000–20,000 ft) from the Ottoman forts guarding the entrance to the Dardanelles.

[17] The patrols were uneventful, until December when the Ottomans decided to intensify their naval operations, in view of the failed peace negotiations with Greece.

At 13:26 p.m. Delfin's captain, Stefanos Paparrigopoulos, was informed that the Ottoman light cruiser Mecidiye had appeared in front of the straits.

On 3 December, the submarine was conducting an early morning patrol, at 8:10 a.m. she received information that the Ottomans began assembling in front of the Dardanelles.

Paparrigopoulos sent a second strongly worded complaint to the fleet commander, Pavlos Kountouriotis, explaining that the stand by orders had made the submarine's successful operation impossible.

[18] On the morning of 9 December, Delfin was approaching the port of Tenedos in order to charge her batteries when she received information that Mecidiye and five Ottoman destroyers had been spotted outside the straits.

Despite the fact that the batteries were at half of their capacity, Delfin sailed at full speed towards the island of Mavria and halted there.

She submerged at 9:52 a.m., but the submarine's bow took on a dangerous down angle of at least 20 degrees during the dive and blowing of all her ballast tanks brought her back to the surface.

In January 1913, Delfin began patrolling the Saronic Gulf in order to protect military transport ships from a possible attack by the Ottoman cruiser Hamidiye.

The Entente intervened into Greek affairs numerous times in an effort to support the pro-war Provisional Government of National Defence, including a blockade of Piraeus and the Salamis Naval Base.

[26] On 28 September 1916, two steamboats from the British battleship HMS Duncan towed Delfin to Adamantas on the island of Milos where the submarine was placed under the supervision of the French Navy.

Delfin resumed operation on 20 May and remained in service until 23 July 1919, when maneuvers were halted due to the corrosion of the ballast tanks.

[27] The poor condition of the hulls and ballast tanks of the submersibles along with the absence of radios and gyrocompasses led to their decommissioning in early 1920.

One of the periscopes and some of the electric motors were used for training purposes in the Hellenic Naval Academy, while the pressure hulls were sold to the Port of Piraeus Authority.