The Battle of Lemnos (Greek: , Turkish: Mondros Deniz Muharebesi), fought on 18 January [O.S. 5 January] 1913, was a naval battle during the First Balkan War, in which the Greeks defeated the second and last attempt of the Ottoman Empire to break the Greek naval blockade of the Dardanelles and reclaim supremacy over the Aegean Sea. This, the final naval battle of the First Balkan War, forced the Ottoman Navy to retreat to its base within the Dardanelles, from which it did not venture for the rest of the war, thus ensuring the dominion of the Aegean Sea and the Aegean islands by Greece.

The First Balkan War (Serbian: Први балкански рат, Prvi Balkanski rat; Bulgarian: Балканска война; Greek: Αʹ Βαλκανικός πόλεμος; Turkish: Birinci Balkan Savaşı) lasted from October 1912 to May 1913 and involved actions of the Balkan League (the Kingdoms of Bulgaria, Serbia, Greece and Montenegro) against the Ottoman Empire. The Balkan states' combined armies overcame the initially numerically inferior (significantly superior by the end of the conflict) and strategically disadvantaged Ottoman armies and achieved rapid success.

The war was a comprehensive and unmitigated disaster for the Ottomans, who lost 83% of their European territories and 69% of their European population. As a result of the war, the League captured and partitioned almost all of the Ottoman Empire's remaining territories in Europe. Ensuing events also led to the creation of an independent Albania, which angered the Serbs. Bulgaria, meanwhile, was dissatisfied over the division of the spoils in Macedonia, and attacked its former allies, Serbia and Greece, on 16 June 1913 which provoked the start of the Second Balkan War.