The Royal Netherlands East Indies Army (Dutch: Koninklijk Nederlands Indisch Leger; KNIL, pronounced [knɪl]; Indonesian: Tentara Kerajaan Hindia Belanda) was the military force maintained by the Kingdom of the Netherlands in its colony of the Dutch East Indies, in areas that are now part of Indonesia. The KNIL's air arm was the Royal Netherlands East Indies Army Air Force. Elements of the Royal Netherlands Navy and Government Navy were also stationed in the Netherlands East Indies.

The KNIL was formed by royal decree on 14 September 1814. It was not part of the Royal Netherlands Army, but a separate military arm specifically formed for service in the Netherlands East Indies. Its establishment coincided with the Dutch drive to expand colonial rule from the 17th century area of control to the far larger territories constituting the Dutch East Indies seventy years later.^{[failed verification]}

The KNIL was involved in many campaigns against indigenous groups in the area including the Padri War (1821–1845), the Java War (1825–1830), crushing the final resistance of Bali inhabitants to colonial rule in 1849, and the prolonged Aceh War (1873–1904). In 1894, Lombok and Karangasem were annexed in response to reports of the local Balinese aristocracy oppressing the native Sasak people. Bali was finally taken under full control with the Dutch intervention in Bali (1906) and the final Dutch intervention in Bali (1908).

In the nineteenth and early twentieth centuries, the KNIL resumed the conquest of the Indonesian archipelago. After 1904 the Netherlands East Indies were considered pacified, with no large-scale armed opposition to Dutch rule until World War II, and the KNIL served a mainly defensive role protecting the Dutch East Indies from the possibility of foreign invasion.

Once the archipelago was considered pacified the KNIL was mainly involved with military policing tasks. To ensure a sizeable European military segment in the KNIL and reduce costly recruitment in Europe, the colonial government introduced obligatory military service for all resident male conscripts in the European legal class in 1917. In 1922 a supplemental legal enactment introduced the creation of the Home Guard (Dutch: Landstorm) for European conscripts older than 32.

Dutch forces in the Netherlands East Indies were severely weakened by the defeat and occupation of the Netherlands itself, by Nazi Germany, in 1940. The KNIL was cut off from external Dutch assistance, except by Royal Netherlands Navy units. The KNIL, hastily and inadequately, attempted to transform into a modern military force able to protect the Dutch East Indies from foreign invasion. By December 1941, Dutch forces in Indonesia numbered around 85,000 personnel: regular troops consisted of about 1,000 officers and 34,000 enlisted soldiers, of whom 28,000 were indigenous. The remainder were made up of locally organised militia, territorial guard units and civilian auxiliaries. The KNIL air force, Militaire Luchtvaart KNIL (Royal Netherlands East Indies Air Force; ML-KNIL) numbered 389 planes of all types, but was largely outclassed by superior Japanese planes. The Royal Netherlands Navy Air Service, or MLD, also had significant forces in the NEI.

During the Dutch East Indies campaign of 1941–42, by forces of the Empire of Japan, most of the KNIL and other Allied forces were quickly defeated. Most European soldiers, which in practice included all able bodied Indo-European males, were interned by the Japanese as POWs. 25% of the POWs did not survive their internment.

A handful of soldiers, mostly indigenous personnel, mounted guerrilla campaigns against the Japanese. These were usually unknown to, and unassisted by, the Allies until the end of the war.