Vormsi (Swedish: Ormsö) is the fourth-largest island of Estonia, measuring 92 square kilometres (36 square miles) and a registered population of around 400. It is located in the Baltic Sea, northwest of the mainland and east of Hiiumaa island, and is part of the West Estonian archipelago (Moonsund archipelago).

Administratively the island forms the main part of the Vormsi vald (rural municipality).

The documented history of Vormsi dates back as far as the 13th century. During most of this time, the island was inhabited by Estonian Swedes ("rannarootslased" in Estonian or "coastal Swedes" in English). Ormsö in Swedish means "Orm's island", and can be translated also as "Snake island". The Estonian name Vormsi and German Worms have both been derived from Ormsö.

The island's population reached 3,000 before World War II. During the war, nearly all of Vormsi's population, along with other Swedes living in Estonia, forming the larger region of Aiboland, were evacuated, or fled, to Sweden.

The villages on the island include: Hullo (the administrative centre), Sviby (the main port), Söderby, Norrby, Diby, Rälby, Förby, Borrby, Kärrslätt, Saxby, Suuremõisa (Magnushof, formerly Busby), Rumpo (the main beach area) and Hosby.

Vormsi is the per capita wealthiest island and region in Estonia outside of the Tallinn metro area, with incomes on the same level as inner Tallinn. This is attributed to the fact that many comparatively wealthy people own summer homes on the island and are registered as residents, so their yearly income tax is forwarded to the island's budget.

Vormsi has only small businesses with no significant industrial activity. The island earns the majority of its revenues from the taxation on the salaries earned by its residents on the Estonian mainland.

The island of Vormsi has homeowners from several countries and regions in Estonia, Sweden, Finland, Switzerland and elsewhere.

• Kanarbik, Madis, Ormsö. De estlandssvenska böndernas kamp mot godsägarna under 1700- och 1800-talet [Nordistica Tartuensia; 9] (Tartu, 2003)
• Beyer, Jürgen, 'Whom should one thank for a narrow escape? Lessons drawn from a perilous journey from Vormsi and Noarootsi to Finland in 1796', Pro Ethnologia 17 (2004), 175-95

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  Vormsi harbour (2015)
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  Paddock, Rälby
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  Old windmill, Rälby
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  Woodsman's cottage, Hullo
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  Beach area in Rumpo
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  Viewing platform in Rumpo peninsula, looking towards Sviby harbor
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  Vormsi cemetery
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  Aurora borealis in Saxby
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  Windmills in Hullo, 1930s
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  Aerial photo of the Borrby village, 1934
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  Swedish communist daily Ny Dag headline on 5 September 1940: "The Swedes on Vormsi greet Stalin and the new regime"
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  Fleeing the Soviet occupation to Sweden, 1944
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  Road signs in Hullo
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  The 15th century Swedish (now Lutheran) church in Hullo (2011)
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  Round crosses in the Vormsi cemetery
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  A Nordic cross flag used in Vormsi
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  Vormsi is connected with the mainland by MS Ormsö (:et)

• Vormsi vald (Vormsi municipality)