Mount Rinjani or Gunung Rinjani is an active volcano in Indonesia on the island of Lombok. Administratively the mountain is in the Regency of North Lombok, West Nusa Tenggara (Indonesian: Nusa Tenggara Barat, NTB). It rises to 3,726 metres (12,224 ft), making it the second highest volcano in Indonesia. On the top of the volcano is a 6 kilometres (3.7 mi) by 8.5 kilometres (5.3 mi) caldera, which is filled partially by the crater lake known as Segara Anak (Child of the Sea). This lake is approximately 2,000 metres (6,600 ft) above sea level and estimated to be about 200 metres (660 ft) deep; the caldera also contains hot springs. A massive eruption of Rinjani in 1258 CE may have triggered the Little Ice Age.

The volcano and the caldera are protected by the Gunung Rinjani National Park established in 1997. Tourism is increasingly popular with trekkers able to visit the rim, make their way into the caldera or even to make the more arduous climb to the highest point; fatalities, however, are not uncommon. In July 2009 the summit route was closed due to volcanic activity at that time and subsequently reopened when the activity decreased. During early 2010 up to and including May 2010 access to Rinjani was at times again restricted due to volcanic activity.

The park is popular for mountain climbs and trekking and represents an important nature reserve and water catchement area. The park is officially 41,330 hectares within the park boundaries and includes a further 66,000 hectares of protected forest outside. The mountain and its satellites form the Mount Rinjani National Park (Taman Nasional Gunung Rinjani) - officially 41,000 hectares within the park boundaries and a further 66,000 hectares of protected forest outside. In 2008, the Indonesian government proposed to UNESCO that Mount Rinjani be one of the world's official geoparks. If this was approved by UNESCO, Mount Rinjani would become the first such geological park in Indonesia.

It has been claimed that the preliminary documentation required for UNESCO registration has not received sufficient support from the Nusa Teggara Barat government offices. Among the requirements to become a geo-park sufficient information must be supplied to show that the location has sufficient and appropriate management, information services, access to educational instruction to facilitate "knowledge-based geotourism", the implementation of a sustainable regional economy, biodiversity conservation, and to have established public access to the park area.

Mount Rinjani has obtained the World Legacy Award from Conservation International and Traveller (2004), and was a finalist for Tourism for Tomorrow Awards (2005 dan 2008) from the World Travel Tourism Council (WTTC).