A-0 Geyser, Geyser in Yellowstone National Park, Wyoming, United States.

A-0 Geyser is a hot spring in Yellowstone National Park that periodically ejects water and steam. The feature sits within an area containing numerous similar thermal eruptions that showcase the park's volcanic system.

The name A-0 is unofficial, given because the geyser sits near the dormant A-1 and A-2 geysers, which it does not resemble. The park closed the trail from the Great Fountain Geyser parking area toward the southeast, which once passed near the feature, because of overuse and abuse; using it is now illegal.

The name A-0 is unofficial, taken from the dormant A-1 and A-2 geysers nearby, though A-0 does not resemble them. The geyser belongs to the White Creek Group with Spindle Geyser and Botryoidal Spring. Between A-0 and Botryoidal Spring sits a tiny unnamed geyser erupting about every 10 minutes from several small holes, reaching a few centimetres to 60 cm.

The area is accessible year-round, though visibility varies depending on season and weather conditions. In winter, steam eruptions are particularly visible against cold air, while trails may have snow coverage.

The name A-0 is unofficial and comes from the geyser's proximity to the dormant A-1 and A-2 geysers, which it does not resemble. The RCN location of A-0 has been unintentionally switched with Dilemma Geyser elsewhere in the Lower Geyser Basin.