Kustavi (Swedish: Gustavs) is a municipality in the region of Lounais-Suomi in Finland. It is in the province of Western Finland and is part of the Southwest Finland region. The municipality has a population of 924 (31 January 2019) and covers an area of 770.15 square kilometres (297.36 sq mi) of which 603.45 km2 (232.99 sq mi) is water. The population density is 5.55 inhabitants per square kilometre (14.4/sq mi).
There are over 2,000 isles within the municipal area.
The municipality is a very popular summer resort and contains over 2,800 summer cottages. The population increases tenfold during the summer months. The larger events are Volter Kilpi literature week in July and Salmon market (Lohimarkkinat) in August. The basic services of the municipality include 3 groceries, a liquor store, a library and a bank. There are two ferry connections to Brändö and Iniö.
The municipality is unilingually Finnish even though it is located adjacent to the Swedish speaking Ahvenanmaa-region.