U-15 or U-XV was a U-10-class submarine or U-boat of the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine) during World War I. U-15 was constructed in Germany and shipped by rail to Pola where she was assembled and launched in April 1915. She was commissioned in October 1915. U-15 was the most successful boat of the U-10 class, sinking six ships totaling more than 8,000 gross register tons (GRT). The boat survived the war and was handed over to Italy as a war reparation and scrapped in 1920.

U-15 was constructed at AG Weser in Bremen for the Austro-Hungarian Navy and then shipped by rail in sections to Pola, where the sections were riveted together. Though there is no specific mention of how long it took for U-15's sections to be assembled, a sister boat, the German Type UB I submarine UB-3, shipped to Pola from Germany in mid-April 1915, was assembled in about two weeks.^[8][1] U-15 was launched in April.

U-15 was a small, coastal submarine that displaced 125.5 long tons (127.5 t) surfaced and 140.25 long tons (142.50 t) submerged. She featured a single shaft, a single 59 bhp (44 kW) Daimler diesel engine for surface running, and a single 119 shp (89 kW) electric motor for submerged travel.^[3] U-15 was capable of up to 6.5 knots (12.0 km/h; 7.5 mph) while surfaced and 5.5 knots (10.2 km/h; 6.3 mph) while submerged at a diving depth of up to 50 metres (160 ft). She was designed for a crew of 17 officers and men.^[3]

U-15 was equipped with two 45 cm (17.7 in) torpedo tubes located in the front and carried a complement of two torpedoes. In October 1916, U-15's armament was supplemented with a 37 mm/23 (1.5 in) quick-firing (QF) gun. This gun was replaced by a 47 mm (1.9 in)/33 QF gun in November 1917.^[3]

SM U-15 was commissioned into the Austro-Hungarian Navy on 6 October under the command of Linienschiffsleutnant Friedrich Schlosser. On 28 November, Linienschiffsleutnant Friedrich Fähndrich was assigned to the first of two stints in command of the boat.^[6] On 18 December, Fähndrich and U-15 attacked and sank two Albanian sailing vessels near Lezhë. The Erzen, of 25 GRT, and the Figlio Preligiona, of 80 GRT, were both sunk at position 41°47′N 19°31′E ^[26] .^[9][10][11] After being relieved by Linienschiffsleutnant Franz Rzemenowsky von Trautenegg from late March to early May 1916, Fähndrich resumed command on 10 May.^[6] One week later, on 17 May, U-15 torpedoed and sank the 2,237 GRT Italian steamer Stura in the Adriatic some 18 nautical miles (33 km) east of Brindisi.^[12][2]

The following month, Fähndrich and the crew of U-15 scored their second double kill when they sank the Italian auxiliary cruiser Cittá di Messina (3,495 GRT) and the French destroyer Fourche (745 GRT).^[13][14] While about 20 nautical miles (37 km) east of Otranto on 23 June, U-15 torpedoed and sank Cittá di Messina. The escorting destroyer Fourche began a depth charge attack on U-15 and assumed success when an oil slick appeared on the surface. After the captain of Fourche turned his attentions to the rescue Cittá di Messina's survivors, U-15 launched a single torpedo that struck Fourche amidships and sank her.^[15]

On 25 October, U-15, back under the command of von Trautenegg,^[6] sank the 2,207 GRT Italian steamer Polcevera,^[16] a sister ship to Stura (sunk by U-15 in May).^[17] Polcevera was the last ship sunk by U-15.

From October 1916 to the end of the fighting in November 1918, U-15's activities are unknown. U-15 was at Pola at the end of the war when Austria-Hungary handed her over to Italy.^[18] U-15 was scrapped at Pola by 1920.^[3]