Hat Creek (Achumawi: Hatiwïwi) is a 48.7-mile-long (78.4 km) stream and tributary of the Pit River, which is located in Shasta County of northern California.

The creek rises in two forks on the eastern slopes of Lassen Peak in Lassen Volcanic National Park, and flows northward through Lassen National Forest to its mouth at Lake Britton near Burney, California.

Hat Creek is so named because a surveyor lost his hat there.

The lower three and a half miles of the creek has been designated by the California Department of Fish and Game as a "Wild Trout Waterway," with restrictive catch limits. This area has a similar geology to the chalk streams of southern England, and is considered a classic trout stream. The trout population is made of wild rainbow and brown trout of up to 20 inches (510 mm) in length.

The main towns along Hat Creek are Hat Creek and Old Station, which is closer to Lassen Park. The Hat Creek Radio Observatory is located near the town of Hat Creek.

During the May 19, 1915, eruption of Mount Lassen, a lahar swept down Hat Creek.

There is a small mountain in Lassen National Park where the stream starts. It is square topped and looks like a hat, therein lies the derivation of the name!