Sundbyberg Municipality (Sundbybergs kommun or Sundbybergs stad) is a municipality in Stockholm County in east central Sweden, just north of the capital Stockholm. Sundbyberg is wholly within the Stockholm urban area and has a 100% urban population.
Sundbyberg was detached from Bromma (which since 1916 is in Stockholm Municipality) in 1888 as a market town (köping). It got the title of a city in 1927. In 1949 parts of Solna Municipality and Spånga (when the rest of Spånga was amalgamated into Stockholm) were added. A proposed merger with Solna in 1971 was never implemented, making Sundbyberg, with an area of , the smallest municipality in Sweden, but also the most densely populated. The municipality prefers to call itself a city, which, however, has no legal significance.
Sundbyberg was for a long time only an area of small agriculture value and most of all used as a place to spend summer for rich families in the city. In 1863 almost the entire area was bought by Anders Petter Löfström, including Duvbo Estate, who began building houses there. In 1870 the first industrial plot was sold and from there the town did expand with railroad, houses, industries and community services of all kind. A. P. Löfström also donated to the municipality, all land for roads, streets, parks, school, church and other public areas.
Sundbyberg never became a suburb but a totally independent industrial town on its own.