The Valley of Ten Thousand Smokes is a valley within Katmai National Park and Preserve in Alaska which is filled with ash flow from the eruption of Novarupta on June 6–8, 1912. Following the eruption, thousands of fumaroles vented steam from the ash. Robert F. Griggs, who explored the volcano's aftermath for the National Geographic Society in 1916, gave the valley its name, saying that "the whole valley as far as the eye could reach was full of hundreds, no thousands—literally, tens of thousands—of smokes curling up from its fissured floor."

Prior to the eruption, the area now called the Valley of Ten Thousand Smokes was an unremarkable and unnamed portion of the Ukak River valley. Although never permanently inhabited by humans, it served as a pass for the Alutiiq people, as well as animals such as grizzly bears.

The 1912 eruption was the largest eruption by volume in the 20th century, erupting a magma volume of about 13 cubic kilometers (3.1 cu mi). As many as 14 major earthquakes between ${\displaystyle M_{\text{s}}}$ 6 to 7, and over 100 earthquakes greater than ${\displaystyle M_{\text{s}}}$ 5, resulted from the collapse of the caldera at Mount Katmai and movement within the magmatic plumbing system.

The ash-filled valley covers a 40-square-mile (100 km²) area. The ash can be up to 700 feet (210 m) deep. In places deep canyons have been cut by the River Lethe, allowing observers to see the ash flow strata. Since the ash has cooled, most of the fumaroles are now extinct and despite its name the valley is no longer filled with 'smoke' (i.e. steam). Vegetation still does not grow in the valley. The signs of volcanic activity are still visible on nearby hills.

Katmai's most recent eruption was in 1927, but there have been non-eruptive events as recently as 2003. The Alaska Volcano Observatory still monitors Katmai's activity as part of the Katmai Cluster, where there are five active stratovolcanos within 15 kilometers (9 mi) of Katmai.

Visitors to the valley most commonly arrive via bus along the 20-mile (32 km) road from Brooks Camp, which is the only road in Katmai Park. The valley is a source of creative exploration by photographers and naturalists.

Basement rocks in the area consist of the Late Jurassic Naknek Formation, which is composed of siltstones, and arkosic sandstones 1.7–2 km thick deposited in a marine shelf and submarine fan delta environment during the Oxfordian-Tithonian. Some 20 dioritic and granodioritic Tertiary plutons pierce the formation. Most prevalent though are the Quaternary stratovolcanoes along the Aleutian Arc associated with the Aleutian subduction zone.

The 1912 Novarupta eruption was characterized by three main episodes spanning 60 hours. Episode I included both a rhyolitic ignimbrite and a simultaneous Plinian dispersal of rhyolitic tephra fallout. The ignimbrite consisted of 9 separate packages of pumiceous pyroclastic flow deposits, radiating outward 9 km, and which filled 11 km³ of the valley to a depth of 100–200 m in the upper portion and 35 m in the lower over the span of 16 hours. It is indurated in the upper valley while nonwelded in the lower. The ignimbrite is noted for its fumaroles, phreatic craters, welded tuff, and flattened fiamme. At the same time, a Plinian dispersal of 8.8 km³ of rhyolitic tephra occurred depositing layers A and B. Episode II consisted of a second Plinian dispersal, but one consisting of dacite tephra, depositing 4.8 km³ of layers C and D. Episode III was also a Plinian dispersal of dacite tephra depositing 3.4 km³ of layers F and G.