Tanganyika was a colonial territory in East Africa which was administered by the United Kingdom in various forms from 1916 until 1961. It was initially administered under military occupation. From 20 July 1922, it was formalised into a League of Nations mandate under British rule. From 1946, it was administered by the UK as a United Nations trust territory. It bordered British East Africa to the North East.

Before World War I, Tanganyika formed part of the German colony of German East Africa. It was gradually occupied by forces from the British Empire and Belgian Congo during the East Africa Campaign, although German resistance continued until 1918. After this, the League of Nations formalised control of the area by the UK, who renamed it "Tanganyika". The UK held Tanganyika as a League of Nations mandate until the end of World War II after which it was held as a United Nations trust territory. In 1961, Tanganyika gained its independence from the UK as Tanganyika, joining the Commonwealth. It became a republic a year later. Tanganyika now forms part of the modern-day sovereign state of Tanzania.

The name of the territory was taken from the large lake in its west. Henry Morton Stanley had found the name of "Tanganika", when he travelled to Ujiji in 1876. He wrote that the locals were not sure about its meaning and conjectured that it meant something like "the great lake spreading out like a plain", or "plain-like lake".

The name was chosen by the British with the Treaty of Versailles, and as such the name took effect when Britain was given control of Tanganyika in 1920. Britain needed a new name to replace "Deutsch Ostafrika" or "German East Africa". Various names were considered, including "Smutsland" in honour of General Jan Smuts (denied for being "inelegant"), "Eburnea", "New Maryland", "Windsorland" after the British Royal Family's new family name, and "Victoria" after both the Lake and the Queen. The Colonial Secretary insisted that "a native name prominently associated with the territory" be selected. "Kilimanjaro", analogous to "Kenya", named after the country's highest mountain, and "Tabora", after the town and trading centre near the geographical centre of the country, were proposed and rejected. Then, the deputy undersecretary to the Colonial Secretary proposed "Tanganyika Protectorate" after Lake Tanganyika; the name was modified after a "junior official suggested that 'Territory' was more in accordance with the [League of Nations mandate]" and that was adopted.

The area that made up Tanganyika was commonly visited by Arabic traders who would come to the area to buy enslaved people and smuggle ivory. The island of Zanzibar was even taken as a part of the Sultanate of Oman; when Seyyid Said came to power in 1806, Omani interests in Tanzania began to increase. During the early 19th century, with British support, Oman began developing in the region more closely to prevent French growth in the Indian Ocean and grow Oman's wealth and influence. Trade caravans began venturing further into the continent, connecting the coast and the interior together. In some areas, Islam became adopted by the native peoples, such as the Yao in the south of the country. Islam has continued to be a major religion within the area, with 36% of the Tanzanian population adhering to Islam.

In the second half of the 19th century, European explorers and colonialists travelled through the African interior from Zanzibar. In 1885, the German Empire declared its intent to establish a protectorate in the area, named German East Africa (GEA), under the leadership of Carl Peters. When the Sultan of Zanzibar objected, German warships threatened to bombard his palace. Britain and Germany then agreed to divide the mainland into various spheres of influence, and the Sultan was forced to acquiesce. The Germans brutally repressed the Maji Maji Rebellion of 1905. The German colonial administration instituted an educational program for native Africans, including elementary, secondary, and vocational schools.^{[page needed]}

The German colonial administrations developed the colony through several means. Cultivation of several profitable cash crops such as cotton, sisal, cocoa and coffee was important to developing the colony as these resources were used for German consumers and industry. Sisal was especially valuable to rope production, and was one of German East Africa's largest exports. In 1893 there was only one sisal plantation in the country; by 1913 there were 54. In 1913, the country exported over 20,000 tons of sisal, 30% of total exports. To ensure that these resources could be moved easily, several railways were built. The most important of these was the Mittellandbahn (Central Line, which connected much of the country with the port city of Dar es Salaam. This railroad is still in use today and has since been connected to other railways across the country.

After Germany's defeat during World War I, GEA was divided among the victorious powers under the Treaty of Versailles. Apart from Ruanda-Urundi (assigned to Belgium) and the small Kionga Triangle (assigned to Portuguese Mozambique), the territory was transferred to British control. "Tanganyika" was adopted by the British as the name for its part of the former German East Africa. On 22 March 1921 Belgium ceded the Kigoma District to Tanganyika.