Coos Bay is an estuary located in northern Coos County, Oregon, in the United States. It is formed where the Coos River enters the Pacific Ocean from the east. The bay measures approximately 12 miles long and up to two miles wide, making it the largest estuary completely within the state of Oregon. Geologically, the bay was created over 20,000 years ago at the end of the Last Glacial Maximum when rising sea levels flooded the mouth of the Coos River.
The geography of the bay is characterized by a sharp northward loop that arches back toward the south to meet the Pacific Ocean. Haynes Inlet enters the top of this loop, while South Slough branches off just before the bay's entrance into the ocean. The surrounding watershed covers an area of about 600 square miles.
Coos Bay serves as Oregon's most significant coastal industrial center and international shipping port. It maintains established commercial ties to other major Pacific rim ports, including San Francisco, the Columbia River, and Puget Sound.