Ullensaker is a municipality in Akershus county, Norway. It is part of the traditional region of Romerike. The administrative centre of the municipality is the village of Jessheim. It has a population of approximately 30,000 inhabitants. Norway's largest international airport Oslo Airport, Gardermoen, is located in Ullensaker.

The municipality (originally the parish) is named after the old Ullensaker farm (Old Norse: Ullinshof), since the first church was built there. The first element is the genitive case of the name Ullinn (a sideform of Ullr). The last element was originally hof which means "temple" but this was later (around 1500 AD) changed to aker meaning "acre" or "field".

Ullensaker does not have an heraldic coat-of-arms properly granted. The municipality uses a non-heraldic badge that is from modern times. The logo was adopted on 8 November 1979, and it shows the god Ullr of Norse mythology holding a bow and three arrows. The colors on the logo can vary.

Ullensaker borders with Eidsvoll to the north, Nes to the east, Sørum to the south, and Gjerdrum and Nannestad to the west. The international airport that serves as the main airport of Norway, Oslo Airport, Gardermoen, is located in Ullensaker. The municipality has two major population centres: the villages of Kløfta and Jessheim, the latter one houses the administration center of Ullensaker. There is also a Royal Norwegian Army base at Sessvollmoen as well as a Royal Norwegian Air Force air station, Gardermoen Air Station.