The Koyuk River is a river on the Seward Peninsula of western Alaska, in the United States. The river originates in the interior of the peninsula, at the Lost Jim Lava Flow of the Bering Land Bridge National Preserve, where it flows southeast towards the mouth of Norton Bay on Norton Sound. The native village of Koyuk is located at its mouth. The two major tributaries are the Peace and Salmon rivers; other tributaries include Dime and Sweepstakes.

The Seward Peninsula is a large peninsula on the western coast of the U.S. state of Alaska whose westernmost point is Cape Prince of Wales. The peninsula projects about 200 mi (320 km) into the Bering Sea between Norton Sound, the Bering Strait, the Chukchi Sea, and Kotzebue Sound, just below the Arctic Circle. The entire peninsula is about 210 mi (330 km) long and 90–140 mi (145–225 km) wide. Like Seward, Alaska, it was named after William H. Seward, the United States Secretary of State who fought for the U.S. purchase of Alaska.