Its neighboring municipalities are Ingå to the west, Kirkkonummi to the east, Lohja to the north-west, and Vihti to the north.

Siuntio's motto is "Ota rauhallisesti – Ta det lugnt", which means "take it easy" in Finnish and Swedish respectively.

[8] Apart from burial sites also a prehistoric hill fort, Skällberget, is found opposite to the church.

[10] During the Middle Ages two large manor houses, Suitia and Sjundby, were built in Siuntio.

[15] The National Author of Finland, Aleksis Kivi, lived in Siuntio in Fanjunkars croft from 1864 to 1871.

[19] In 2011 Siuntio, together with the municipalities of Karjalohja, Sammatti and Nummi-Pusula, decided to merge with the city of Lohja starting in 2013.

After several unclear negotiations, Siuntio failed to deliver any kind of answers and therefore was cut out of the merge with Lohja.

The largest urban areas of Siuntio are the new municipal centre Siuntion asemanseutu (lit.

Suitia, together with St. Peter's Church, are listed as Nationally Significant Built Cultural Environments and are protected by the Finnish Heritage Agency.

[27] The Sjundby Manor House was built in the 1560s by Swedish king Gustav Vasa's stall master Jakob Henriksson.

The Adlercreutz family has owned Sjundby since the 18th century and tours inside the castle are arranged during summer.

Writings in Cyrillic script instructing how Soviet soldiers should behave can still be seen on the walls of the Sjundby dairy house.

Sjundby is listed as a Nationally Significant Built Cultural Environment and is protected by the Finnish Heritage Agency.

[28] The Pickala Manor House was on the Soviet side of the border during the leasing of Porkkala Naval Base.

The road was paved with cobblestone to make it strong enough for Soviet military vehicles to drive on.

[29] Pickala Manor House and its surroundings are listed as Nationally Significant Built Cultural Environment and protected by the Finnish Heritage Agency.

The official heraldic explanation of the coat of arms is: In a blue field, a horizontally positioned golden key, with a cross in the shaft and the handle designed as a quadrilateral, between two bars of silver which form wave-like patterns.

In 2020 Siuntio started constructing a new school complex in the municipal centre after Aleksis Kiven koulu was demolished due to it sinking in the ground and its inner air problems.

[36] The new building complex, called Siuntion Sydän – Sjundeå Hjärta, was completed in 2022 and houses the communal library and both schools.