Seaside is a city in Clatsop County, Oregon, United States. The city is named after a popular summer resort called Seaside House built by prominent Portland land developer and railroad builder Ben Holladay in the 1870s. Mail to the area was first addressed to Seaside House, and then later shortened to Sea Side which was eventually transformed into Seaside and adopted as the city's name.

The city has historically been one of the Oregon Coast's most visited and most popular oceanfront resorts even before it was incorporated in 1899.

Located only 80 miles from Portland, the state's largest metropolitan area, Seaside lies on the edge of the Pacific Ocean, at the southern end of the Clatsop Plains, and it is divided by the Necanicum River which flows out to the ocean at the northern edge of town.

The total population is 5,900 based on a 2000 census for the city, according to the United States Census Bureau. However, the city's population grows in the summer due to influx of tourist.

Seaside is famous for its natural wonders and attractions and is a common tourist destination for residents of the Portland area. The city hosts some large event and festivals including sporting competitions particularly during the summertime. These includes an annual volleyball tournament held every August, the Hood to Coast relay race and Portland-to-Coast relay walk, which also take place in August and the Seaside Three-Course Challenge, a cross country race, held at Fort Rilea and is hosted by Seaside High School.