[2] The county is named for the Clatsop tribe of Native Americans, who lived along the coast of the Pacific Ocean prior to European settlement.

Clatsop County comprises the Astoria, OR Micropolitan Statistical Area, or Sunset Empire, and is located in Northwest Oregon.

The Lewis and Clark Expedition stayed for the winter of 1805–06 in the area, establishing Fort Clatsop as one of the earliest American structures on the west coast of North America.

Astoria, Oregon's oldest settlement, was established as a fur trading post in 1811 and named after John Jacob Astor.

Clatsop County was created from the northern and western portions of the original Twality District on June 22, 1844.

Until the creation of Vancouver District five days later, Clatsop County extended north across the Columbia into present-day Washington.

Fort Stevens, located near the peninsula formed by the south shore of the Columbia river and the Pacific Ocean, became the only continental US military installation attacked in World War II, when submarine I-25 of the Imperial Japanese Navy fired 17 rounds at the base on June 21, 1942.

While the damage caused was slight (reportedly only a baseball backstop was damaged and a powerline severed), the presence of the enemy ship sowed panic along the Pacific coast of the United States, and other minor attacks occurred elsewhere in the region, including Vancouver Island.

[6] The highest point is Saddle Mountain at 3,283 feet (1,001 m), part of the Northern Oregon Coast Range.

[7] Clatsop County is home to a number of plant species that are rare or endemic, including Agrostis swalalahos,[8] Cardamine pattersonii, Carex macrochaeta, Castilleja chambersii, Erigeron peregrinus var.

The principal industries of Clatsop County are manufacturing, travel (primarily tourism), and trade.

Like all of the northern Oregon coast, Clatsop County was consistently Republican up to and including the 1928 election,[24] except in 1912 when third-party former President Theodore Roosevelt won by one vote with the lowest percentage for a winning candidate in any county since the Civil War.

[25] Since Franklin D. Roosevelt won the county in 1932, Clatsop has generally been carried by the Democratic candidate in presidential elections.

[28] Clatsop County is located in Oregon's 1st congressional district, which has a Cook Partisan Voting Index of D+18 and is represented by Suzanne Bonamici.