Anak Krakatau is a volcanic island in Indonesia. On 29 December 1927, Anak Krakatau first emerged from the caldera formed in 1883 by the explosive volcanic eruption that destroyed the island of Krakatoa. There has been sporadic eruptive activity at the site since the late 20th century, culminating in a large sector collapse of the volcano, which caused a deadly tsunami in December 2018. There has been subsequent activity since. Owing to its young age the island is one of several in the area that are of interest to, and the subject of extensive study by volcanologists.

After the cataclysmic eruption of Krakatoa in 1883, Krakatoa Island lost approximately two-thirds of its mass on the northwest side, obliterating the peaks of Perboewatan and Danan, and leaving only the southern half of the island, including the Rakata volcano, as the last remnant of the original island. The lost area became a shallow sea.

In early 1927 volcanic activity began to appear at the point located between where the former peaks of Mount Perboewatan and Mount Danan had been. This was a short-lived appearance of a small island that was sunk by sea waves within a week. Several months later volcanic activity began to create a more permanent land formation which, owing to rain and waves, once again collapsed under the sea after its volcanic activity stopped. This process recurred several times during the next three years. On 11 August 1930 the volcanic island permanently rose above sea level and was locally named Anak Krakatau (Child of Krakatoa). It has been the site of repeated eruptive episodes ever since. Anak Krakatau's highest point increased at an average rate of 7–9 meters per year through September 2018.

Anak Krakatau is located in the Sunda Strait—between the islands of Java and Sumatra—in the Indonesian province of Lampung. The volcano is contained within the Ujung Kulon National Park, and is part of the Pacific Ring of Fire.

The island is situated approximately 700 km (430 mi) north of the Sunda Trench marking the subduction zone separating the Australian Plate and the fixed Sunda Plate, atop an oceanic crust of less than 25 km (16 mi) in thickness. In geologic terms, it has recently formed within the caldera of the Krakatoa volcanic eruption. The entire island comprises a Somma-stratovolcano system of the late Holocene epoch, and features a pyroclastic cone. The major rock-type components of Anak Krakatau include andesite, dacite, and basalt; with minor indications of trachyte.

The island had reached a maximum elevation of 338 m (1,109 ft) before its collapse during the 2018 eruptive event.

The volcano's most recent eruptive episode began in 1994. Quiet periods of a few days have alternated with almost continuous Strombolian eruptions since then. Hot gases, rocks, and lava were released in an eruption in April 2008. Scientists monitoring the volcano warned people to stay out of a 3 km (1.9 mi) zone around the island.

On 6 May 2009, the Volcanological Survey of Indonesia raised the eruption alert status of Anak Krakatau to Level 3. An expedition to the volcano revealed that a 100 m (330 ft) wide lava dome was growing in its crater. In January 2012, volcanologists at the University of Oregon warned that a tsunami caused by flank collapse of Anak Krakatau was likely, as it had formed on the steep eastern slope of the large caldera formed by the 1883 explosive eruption.