Arenal Volcano (Spanish: Volcán Arenal) is a recently active andesitic stratovolcano in north-western Costa Rica around 90 km (56 mi) northwest of San José, in the province of Alajuela, canton of San Carlos, and district of La Fortuna. The Arenal volcano stands 1,633 metres (5,358 ft) high. It is conically shaped with a crater 140 metres (460 ft) in diameter. Geologically, Arenal is considered a young volcano and it is estimated to be less than 7,500 years old. It is also known as "Pan de Azúcar", "Canaste", "Volcan Costa Rica", "Volcan Río Frío" or "Guatusos Peak".

The volcano was dormant for hundreds of years and exhibited two craters at its summit, with minor fumaroles activity, covered by dense vegetation. In 1968 it erupted unexpectedly, destroying the small town of Tabacón. Due to the eruption three more craters were created on the western flanks but only one of them exists today. By duration, Arenal's eruption from 1968 to 2010 is the tenth longest volcanic eruption on Earth since 1750. Since 2010, Arenal has been dormant.

Arenal is one of seven historically active Costa Rican volcanoes along with Poás, Irazú, Miravalles, Orosí, Rincón de la Vieja complex, and Turrialba. It was Costa Rica's most active volcano until 2010, and one of the ten most active volcanoes in the world. It has been studied by seismologists for many years. The volcano is located at the center of Arenal Volcano National Park in the northern zone of the country, 15 kilometres (9.3 mi) southwest of the La Fortuna district in San Carlos (canton), Costa Rica.

Arenal Volcano area is an important watershed for the Arenal Lake Reservoir. The reservoir's water is used for hydroelectric power. It is also connected to the national system.

Arenal has several eruptive vents. Chato is a dormant stratovolcanic cone. It is believed that Chato first erupted 38,000 years ago during the Pleistocene period and last erupted about 3,500 years ago. Chatito is a lava dome with an elevation of 1,100 metres (3,609 ft). Espina is another lava dome.

A study published in 2022 analyzed the long-term eruptive behavior of Arenal Volcano, which remained active from 1968 until 2010. The study identified Arenal as an open-vent system, meaning that magma was consistently supplied to the surface without major explosive events. Researchers examined how magma ascent, degassing, and structural changes influenced the volcano's eruptive patterns. The study also suggested that while Arenal is currently dormant, future activity is likely due to its persistent magmatic supply.

Arenal is the youngest of all the volcanoes in Costa Rica. Scientists have been able to date its activity back more than 7000 years. The area remained largely unexplored until 1937, when a documented expedition took place to reach the summit. It was eruptive from 1968 until 2010.

On Monday, July 29, 1968, at 7:30 a.m., the Arenal Volcano suddenly and violently erupted. The eruptions continued unabated for several days, burying over 15 square kilometers (5.8 sq mi) under rocks, lava and ash. When it was finally over, the eruptions had killed 87 people and buried 3 small villages – Tabacón, Pueblo Nuevo and San Luís – and affected more than 232 square kilometers (90 sq mi) of land. Crops were spoiled, property was ruined, and large numbers of livestock were killed.