Argostoli (Greek: Αργοστόλι, Katharevousa: Αργοστόλιον) is a town and a former municipality on the island of Kefalonia, Ionian Islands, Greece. Since the 2011 local government reform it is part of the municipality Kefalonia, of which it is a municipal unit. It has been the capital and administrative centre of Kefalonia since 1757, following a population shift down from the old capital of Agios Georgios (also known as Kastro) to take advantage of the trading opportunities provided by the sheltered bay upon which Argostoli sits. Argostoli developed in to one of the busiest ports in Greece, leading to prosperity and growth. The 2001 census recorded a population of 12,589 in the Argostoli municipal unit. Its largest towns are Argostóli (pop. 9,037), Dilináta (739), Farakláta (411), and Kardakáta (362).

To the east of Argostoli, at the end of the bay, beneath the aforementioned Castle of St. George, sits the Koutavos Lagoon, a feeding ground for the Loggerhead turtles (Caretta caretta). Now a nature reserve, the Koutavos Lagoon was once an almost impassable swamp where mosquitoes and malaria were rife. Under the British governor of the island, General Sir Charles James Napier, a wooden bridge was constructed across the lagoon in 1813 by Colonel Charles Philip de Bosset, a Swiss engineer in the employ of the British army.