Seaside is a city in Clatsop County, Oregon, United States. The name Seaside is derived from Seaside House, a historic summer resort built in the 1870s by railroad magnate Ben Holladay. The city's population was 5,900 at the 2000 census.
About January 1, 1806, a group of men from the Lewis and Clark Expedition built a salt-making cairn at the present site of Seaside. The Native American name for the Clatsop village near the cairn was Ne-co-tat. The city was incorporated on February 17, 1899.
In 1912, Alexandre Gilbert (1843–1932) was elected Mayor of Seaside. Gilbert was a French immigrant, a veteran of the Franco Prussian War. After living in San Francisco, California and Astoria, Oregon, Gilbert moved to Seaside where he had a beach cottage (built in 1885). Gilbert was a real estate developer who donated land to the City of Seaside for its one and a half mile long Promanade, or "Prom," along the Pacific beach.