Mariehamn is the capital of the Åland Islands, an autonomous territory under Finnish sovereignty. Mariehamn is the seat of the Government and Parliament of Åland, and 40% of the population of Åland live in the city. It is mostly surrounded by Jomala, the second largest municipality in Åland in terms of population; to the east it is bordered by Lemland. Like all of Åland, Mariehamn is unilingually Swedish-speaking and around 88% of the inhabitants speak it as their native language.

The town was named after the Russian empress Maria Alexandrovna, literally meaning "Marie's Port". Mariehamn was founded in 1861, around the village of Övernäs, in what was at the time part of the municipality of Jomala. The city has since expanded and incorporated more of Jomala territory. Mariehamn was built according to a very regular scheme which is well-preserved. One of the oldest streets is Södragatan where many wooden houses dating from the 19th century can be seen. Following the First World War Mariehamn was home to the Mariehamn Grain Fleet.

The city is located on a peninsula. It has two important harbours, one located on the western shore and one on the eastern shore, which are ice-free for nearly the whole year, and have no tides. The Western Harbour is an important international harbour with daily traffic to Sweden, Estonia and mainland Finland. A powerful incentive for Baltic ferries to stop at Mariehamn is that, with respect to indirect taxation, Åland is not part of the EU customs zone and so duty-free goods can be sold aboard. Åland and Mariehamn have a reputable heritage in shipping. The Flying P-Liner Pommern museum ship (part of the Åland Maritime Museum) is anchored in the Western Harbour. The Eastern Harbour features one of the largest marinas in Scandinavia. The famous Dutch steamer Jan Nieveen (now called F.P. von Knorring, after Åland teacher and vicar Frans Peter von Knorring) can also be found here.