Sipoo (Finnish pronunciation: [ˈsipoː], Swedish: Sibbo) is a municipality of Finland. Its seat is in Nikkilä/Nickby. It is the eastern neighbour of Helsinki and is located in the Uusimaa region. The municipality has a population of 18,510 (31 January 2012) and covers an area of 698.59 square kilometres (269.73 sq mi) of which 358.97 km² (138.60 sq mi) is water. The population density is 54.5 inhabitants per square kilometre (141 /sq mi).

The once almost completely Swedish-speaking municipality is since 1953 bilingual, a majority being Finnish speakers since 2003, due to migration from other parts of Finland. Today the Finnish-speaking majority stands for ca 60% and the Swedish-speaking minority for about 39% of the population.

On June 26, 2006, the Sipoo town council decided on a strategy to triple the municipality's population over the next 25 years. The decision was made after Helsinki announced plans to annex a part of the municipality in order to continue to build high-end urbanizations in the coastline (and thus high-tax revenue producing). The Finnish Council of State voted in favour of the annexation on June 28, 2007, with votes 8 to 4. Sipoo disputed this and appealed the Supreme Administrative Court but the court upheld the decision of the Council of State and the annexation took place on January 1, 2009. As Helsinki did not directly border Sipoo at any point, the city of Vantaa ceded the area lain between Helsinki and Sipoo to Helsinki in the process.