Kustavi (Finnish pronunciation: [ˈkustɑʋi]; Swedish: Gustavs) is a municipality of Finland. It is in the province of Western Finland and is part of the Southwest Finland region. The municipality has a population of 965 (30 June 2025), which makes it the smallest municipality in southwest Finland in terms of population. It 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 5.82 per square kilometre (15.1/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 in the municipality include three 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 Åland-region. The municipality is named after King Gustav III of Sweden.

Media related to Kustavi at Wikimedia Commons