Map - Third Siege of Messolonghi (Mesolóngi)

The Third Siege of Missolonghi (Τρίτη Πολιορκία του Μεσσολογίου, often erroneously referred to as the second siege) was fought in the Greek War of Independence, between the Ottoman Empire and the Greek rebels, from 15 April 1825 to 10 April 1826. The Ottomans had already tried and failed to capture the city in 1822 and 1823, but returned in 1825 with a stronger force of infantry and a stronger navy supporting the infantry. The Greeks held out for almost a year before they ran out of food and attempted a mass breakout, which however resulted in a disaster, with the larger part of the Greeks slain. This defeat was a key factor leading to intervention by the Great Powers who, hearing about the atrocities, felt sympathetic to the Greek cause.

Missolonghi is a town in southern Aetolia-Acarnania in western Continental Greece, located on a promontory jutting into a lagoon at the entrance of the namesake gulf. The town emerged as a fishing and trading settlement sometime in the 16th century. When the Greek War of Independence broke out in spring 1821, Missolonghi was the first place in western Greece to join the uprising, on 20 May 1822, led by the town elders, such as Athanasios Razikotsikas. With the aid of the klepht chieftain Dimitrios Makris, the nearby island of Anatoliko was also captured soon after.

Its location made it a vital bastion to the Greeks in the War of Independence: protected by a chain of small islands and its lagoon from the sea, and by a wall and the marshy terrain from the landward side, it was strategically located near the Peloponnese and the Ionian Islands. In November 1821, Alexandros Mavrokordatos established a regional government for the territories controlled by the revolutionaries in western Greece, the "Senate of Western Continental Greece".

The town's fortifications were initially limited to a ditch 2 m wide and 1.2 m deep, in many places filled up with rubbish, as well as by a small wall, not higher than 1 m and in need of repair, with fourteen guns. Nevertheless, the city held out against the first Ottoman attempt to capture it in 1822. A 7,000–8,000 strong Ottoman army under Omer Vrioni and Mehmed Reshid Pasha laid siege to the city on 25 October 1822. The small Greek garrison of 500 men managed to delay the Ottomans by pretending to negotiate a surrender until the Greek fleet landed reinforcements on 8 November. The subsequent Ottoman attacks were beaten off, and the onset of winter, disease, and the attacks of other Greek forces from the rear under Georgios Karaiskakis forced the Ottoman commanders to lift the siege on 31 December 1822.

A second Ottoman attack, led by Vrioni and Mustafa Pasha of Scutari, was launched on 20 September 1823, and focused mostly on Anatoliko. Facing the onset of winter, disease, the failure of the simultaneous Ottoman operations in eastern Greece, and Greek attacks on their foraging parties, the Ottoman commanders abandoned the siege on 17 November. In April 1824, Lord Byron died in Missolonghi of an illness, adding to the fame of the city.