The 'Action off Bastia' was an Italian naval attack off Bastia, the principal port of Corsica in the Tyrrhenian Sea, on German vessels (9 September 1943).

This was one of the few successful Italian reactions to the German 'Achse' (ii) occupation of Italy, and one of the first acts of resistance by the Italian armed forces against Germany after the Cassibile armistice that ended hostilities between Italy and the Allied powers.

When the armistice between Italy and the Allied forces was announced, on the evening of 8 September 1943, the harbour of Bastia in Italian-occupied French island of Corsica, accommodated both Italian and German vessels. At moorings in the harbour were the Italian 'Ciclone' class torpedo boats Ardito and Aliseo, the Italian merchant ships Sassari and Humanitas, and a small German flotilla which included the submarine chasers UJ 2203 (former French survey vessel Austral) and UJ 2219 (former Belgian yacht Insuma and five Marinefährprahme ferry craft (F 366, F 387, F 459, F 612 and F 623). The 'Gabbiano' class corvette Cormorano was at the time patrolling off Bastia.

The local Italian and German commanders soon reached an informal agreement whereby the German forces would be allowed to return safely to the Italian mainland. Meanwhile, however, the German forces secretly prepared to launch a surprise attack to seize the Italian ships moored in the harbour. This attack began at 23.45 on 8 September, when two groups of German soldiers, after hearing the whistle that signalled the attack, stormed Ardito, which was heavily damaged and had 70 of her 180-man crew killed or taken prisoner, and the German also seized the merchant ships Sassari and Humanitas. On board the latter, German gunners manning the anti-aircraft guns turned their weapons on the Italian crew and embarked soldiers, and the Italian look-outs were stabbed or killed with hand grenades.

Commanded by Capitano di Fregata Carlo Fecia de Cossato,Aliseo had just departed the harbour when the German attack began. Shortly after dawn on 9 September, a combat group of the 10o Raggruppamento Celere Bersaglieri launched a counterattack and recaptured the port, Ardito, Sassari and Humanitas. The German flotilla was ordered to leave the harbour, but as soon as it did so, its vessels were shelled by the 75-mm (2.95-in) guns of the Italian coastal batteries, which damaged UJ 2203 and some of the Marinefährprahme.

Aliseo was then ordered by the port commander to attack and destroy the German vessels. Shortly after 07.00 the German flotilla, heading in a column led by UJ 2203, opened fire on Aliseo, which started to return fire at 07.06 at a range of 9,075 yards (8300 m). At 07.30 Aliseo was hit by an 88-mm (3.465-in) shell in the engine room and left dead in the water, but the damage was quickly repaired and the torpedo boat closed and engaged her adversaries in succession, destroying them one after the other: at 08.20 UJ 2203, after suffering several hits, blew up; 10 minutes later UJ 2219 was also destroyed when her magazines exploded; and between 08.30 and 08.35 Aliseo also sank F 366, F 459 and F 623. Cormorano became involved during the final phase of the action and, with Aliseo, forced F 387 and F 612 to run aground, after which they were abandoned and destroyed. Cormorano also sank FL B-412, a Luftwaffe service motor boat.

Some 25 German survivors were recovered by Aliseo, which then with the damaged Ardito made for Portoferraio, on the Italian mainland to the east of Corsica, as ordered. The damage gad had suffered meant that Ardito was later left behind in Portoferraio, where she was eventually captured by German forces.