Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is 96 miles (154 km) south of Seattle, Washington and 50 miles (80 km) northeast of Portland, Oregon.

Mount St. Helens takes its English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey of the area in the late 18th century. The volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.

Mount St. Helens is most notorious for its catastrophic eruption on May 18, 1980, at 8:32 am PDT, the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale, caused an eruption, reducing the elevation of the mountain's summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m) and replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied.

As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.

In 1982, President Ronald Reagan and the U.S. Congress established the Mount St. Helens National Volcanic Monument, a 110,000 acres (45,000 ha) area around the mountain and within the Gifford Pinchot National Forest.

Following the 1980 eruption, the area was left to gradually return to its natural state. In 1987, the U.S. Forest Service reopened the mountain to climbing. It remained open until 2004 when renewed activity caused the closure of the area around the mountain (see Geological history section above for more details).

Most notable was the closure of the Monitor Ridge trail, which previously let up to 100 permitted hikers per day climb to the summit. On July 21, 2006, the mountain was again opened to climbers. In February 2010, a climber died after falling from the rim into the crater.

The mountain is now circled by the Loowit Trail at elevations of 4000–4900 feet. The northern segment of the trail from the South Fork Toutle River on the west to Windy Pass on the east is a restricted zone where camping, biking, pets, fires, and off-trail excursions are all prohibited.