Semeru, Active stratovolcano in East Java, Indonesia.

Semeru is a stratovolcano in East Java, Indonesia, whose Mahameru summit reaches 3,676 metres, the highest point on Java. Its upper slopes are bare grey-brown rock cut by narrow vertical gullies and ribs, while dense low vegetation covers the lower flanks. A lower rounded shoulder sits to the right of the summit cone.

Since 1818, at least 61 eruptive periods have been recorded, 11 of them fatal, almost all with a Volcanic Explosivity Index of 2 or 3. Semeru has erupted almost continuously since 1967. In December 2021, ejections reached 11 km, lava flowed 5–11.5 km and ash fell up to 30 km away; at least 57 people died and over 10,000 were displaced.

In Javanese tradition, the mountain is seen as the home of the gods and remains a place of deep respect for many local communities. Visitors walking through the surrounding area may come across small offerings placed along the trail, a practice that continues to this day.

A permit from the Bromo Tengger Semeru National Park office is required to hike, and registration with local authorities is also necessary before setting out. Conditions on the mountain can change fast due to weather and volcanic activity, so checking local advisories on the day of your visit is a good habit.

The summit crater, Jonggring Saloko, erupts glowing material and ash every 15 to 30 minutes, and in November 1997 alone Semeru erupted 2,990 times. Locals call the hot gas clouds wedhus gembel, Javanese for "shaggy goat", after their fleece-like look.