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is a city in Clatsop County, Oregon, United States. The name Seaside came from a summer resort built by the railroad magnate Ben Holladay in the 1870s, Seaside House, located a mile south of the business center. The population was 5,900 at the 2000 census.
According to the United States Census Bureau, the city has a total area of 4.0 square miles (10.4 km²), of which, 3.9 square miles (10.0 km²) of it is land and 0.2 square miles (0.4 km²) of it (3.99%) is water.
Seaside lies on the edge of the Pacific Ocean, at the southern end of the Clatsop Plains, about 29 km (18 mi) south of where the Columbia River empties into the Pacific. The Necanicum River bisects the town and flows out to the ocean at the northern edge of town. Tillamook Head towers over the south edge of town.