The Arenal Volcano, in Spanish Volcán Arenal, is an active andesitic stratovolcano in north-western Costa Rica (10.5N, 84.7W), around 90 km north-west of San José, in the province of Alajuela, canton of San Carlos, and district of La Fortuna. It is the most active volcano in Costa Rica. Arenal is the youngest and most active of all the mountains in Costa Rica. Scientists have been able to date it back to more than 4000 years ago. The area remained largely unexplored until 1937, when a documented expedition took place to reach the summit.

Once locally known as Cerro Arenal (i.e. Mount Arenal) it was presumed extinct until July 29, 1968 when an earthquake caused it to erupt after approximately 400 years of dormancy. The eruption wiped out the town of Arenal and killed 87 people. It lasted for several days, and during that time destroyed crops, property, livestock, and forests. 15 square kilometers were buried and the explosion affected over 232 square kilometers. The eruption caused three new and active craters to form. Before the eruption, there was a wide variety of plant and wildlife on the mountain. It has been active since the explosion and can be reported to have minor eruptions every 5-10 minutes.

Arenal rises 1657 metres above sea level and overlooks Lake Arenal; both are part of Arenal Volcano National Park. The country's most active volcano, Arenal's eruptions are strombolian in type, being frequent but moderate and can be viewed without danger as long as the viewers are behind the safety perimeter. This constant activity and minimal endangerment makes Arenal a popular tourist destination, especially at night (See image), because the lava and incandescent rocks make a spectacular sight.