The Lesser Sunda Islands (Indonesian: Kepulauan Sunda Kecil, Tetum: Illá Sunda ki'ik sirá, Balinese: ᬓᬧᬸᬮᭀᬯᬦ᭄ᬲᬸᬦ᭄ᬤᬘᬾᬦᬶᬓ᭄, romanized: Kapuloan Sunda cénik), now known as Nusa Tenggara Islands (Indonesian: Kepulauan Nusa Tenggara, or "Southeast Islands"), are an archipelago in the Indonesian archipelago. Most of the Lesser Sunda Islands are located within the Wallacea region, except for the Bali province which is west of the Wallace Line and is within the Sunda Shelf. Together with the Greater Sunda Islands to the west, they make up the Sunda Islands. The islands are part of a volcanic arc, the Sunda Arc, formed by subduction along the Sunda Trench in the Java Sea. In 1930 the population was 3,460,059; today over 17 million people live on the islands. Etymologically, Nusa Tenggara means "Southeast Islands" from the words of nusa which means 'island' from Old Javanese language and tenggara means 'southeast'.

The main Lesser Sunda Islands are, from west to east: Bali, Lombok, Sumbawa, Flores, Sumba, Savu, Rote, Timor, Atauro, Alor archipelago, Barat Daya Islands, and Tanimbar Islands. Apart from the eastern half of Timor island and Atauro island which constitute the nation of Timor Leste, all the other islands are part of Indonesia.

The Lesser Sunda Islands consist of two geologically distinct archipelagos. The northern archipelago, which includes Bali, Lombok, Sumbawa, Flores and Wetar, is volcanic in origin. A number of these volcanoes, like Mount Rinjani on Lombok, are still active while others, such as Ilikedeka on Flores, are extinct. The northern archipelago began to be formed during the Pliocene, about 15 million years ago, as a result of the collision between the Australian and the Asian plates. The islands of the southern archipelago, including Sumba, Timor and Babar, are non-volcanic and appear to belong to the Australian plate. The geology and ecology of the northern archipelago share similar history, characteristics, and processes with the southern Maluku Islands, which continue the same island arc to the east.

There is a long history of geological study of these regions since Indonesian colonial times; however, the geological formation and progression is not fully understood, and theories of the geological evolution of the islands changed extensively during the last decades of the 20th century.

Lying at the collision of two tectonic plates, the Lesser Sunda Islands comprise some of the most geologically complex and active regions in the world. The province of Bali is the only part of Nusa Tenggara located on the Sunda Shelf and that is not within the Wallacea region and that is west of the Wallace Line.

There are a number of volcanoes located on the Lesser Sunda Islands.

The Lesser Sunda Islands differ from the large islands of Java or Sumatra in consisting of many small islands, sometimes divided by deep oceanic trenches. Movement of flora and fauna between islands is limited, leading to the evolution of a high rate of localized species, most famously the Komodo dragon. As described by Alfred Wallace in The Malay Archipelago, the Wallace Line passes between Bali and Lombok, along the deep waters of the Lombok Strait which formed a water barrier even when lower sea levels linked the now-separated islands and landmasses on either side. The islands east of the Lombok Strait are part of Wallacea, and are thus characterised by a blend of wildlife of Asian and Australasian origin in this region. Asian species predominate in the Lesser Sundas: Weber's Line, which marks the boundary between the parts of Wallacea with mainly Asian and Australasian species respectively, runs to the east of the group. These islands have the driest climate in Indonesia, and tropical dry broadleaf forests are predominant, in contrast to the tropical moist forests that prevail in most of Indonesia.

The Lesser Sunda Islands are divided among six ecoregions: