It is believed that modern humans originate from the rift valley region of East Africa, and as well as fossilized hominid remains, archaeologists have uncovered Africa's oldest human settlement in Tanzania.

In 1498, Vasco da Gama, the Portuguese explorer, became the first European to visit the Tanzanian coast; in 1502, on his second visit there, he made Kilwa tributary. In 1505, Kilwa was sacked by Francisco d’Almeida, another Portuguese explorer, and by 1506 Portugal controlled most of the coast of E Africa. The Portuguese did not cooperate with the local people, and their impact was mostly negative—trade was disrupted, towns declined, and people migrated from the region. However, Kilwa’s trade seems to have grown as a result of contact with the Portuguese. Toward the end of the 16th cent., the Zimba, a group from SE Africa, moved rapidly up the coast, causing considerable damage; in 1587 they sacked Kilwa and killed about 3,000 persons (roughly 40% of its inhabitants). In 1698 the Portuguese were expelled from the E African coast (except for a brief return in 1725) with the help of Arabs from Oman. In the early 18th cent., the Omanis showed some interest in the commerce of E Africa, and this increased after the Bu Said dynasty replaced the Yarubi rulers in 1741. Oman’s commercial activity was centered on Zanzibar (and, to a lesser extent, at Mombasa), from which it controlled the overseas trade of E Africa. By the early 19th cent. numerous towns on the Tanzanian coast had been founded or revived; these included Tanga, Pangani, Bagamoyo, Kilwa Kivinje (situated on the mainland near Kilwa Kisiwani), Lindi, and Mikandani.

Sayyid Said, the great Bu Saidi ruler, took a great interest in E Africa and in 1841 permanently moved his capital from Muscat, in Oman, to Zanzibar. He brought with him many Arabs, who settled in the mainland towns as well as on Zanzibar. About the same time, new caravan routes into the far interior were opened up; the three main lines went from Kilwa and Lindi to the Lake Nyasa region; from Bagamoyo and Mbwamaji (near present-day Dar-es-Salaam) to Tabora, where one branch continued west to Ujiji (and on into modern Congo) and another went north to the Victoria Nyanza region; and from Pangani and Tanga northwest into modern Kenya via Mt. Kilimanjaro. The caravans following the southern route obtained mainly slaves and ivory; along the more northerly routes ivory was the chief commodity purchased. As a result, the Swahili language (a blend of Bantu grammar and a considerable Arabic vocabulary) and culture gained new adherents. In the middle third of the 19th cent. several European missionaries and explorers visited various parts of Tanzania, notably Mt. Kilimanjaro, Tabora, Lake Victoria, and Lake Nyasa. From the 1860s to the early 1880s Mirambo, a Nyamwezi, headed a large state that controlled much of the caravan trade of central and N Tanzania. About the same time Tippu Tib, a Zanzibari, organized large caravans that passed through Tanzania to present-day Zambia and Congo, where ivory and slaves were obtained.

As the scramble for African territory among the European powers intensified in the 1880s, Carl Peters and other members of the Society for German Colonization signed treaties with Africans (1884–85) in the hinterland of the Tanzanian coast. By an agreement with Great Britain in 1886, Germany established a vague sphere of influence over mainland Tanzania, except for a narrow strip of land along the coast that remained under the suzerainty of the sultan of Zanzibar, who leased it to the Germans. The German East Africa Company (founded 1887) governed the territory, called German East Africa. The company’s aggressive conduct resulted in a major resistance movement along the coast by Arabs, Swahili (whose main leaders were Abushiri and Bwana Heri), and other Africans that was only defeated with the help of the German government. A second Anglo-German agreement (1890) added Rwanda, Burundi, and other regions to German East Africa. Because the company had proved to be an ineffective ruler, the German government in 1891 took over the country (which by then included the coast) and declared it a protectorate. However, it was not until 1898, with the death of the Hehe ruler, Mkwawa, who strongly opposed European rule, that the Germans succeeded in controlling the country. During the period 1905 to 1907 the Maji Maji revolt against German rule engulfed most of SE Tanzania; about 75,000 Africans lost their lives as a result of German military campaigns and lack of food. Under the Germans, several new crops (including sisal, cotton, and plantation-grown rubber) were introduced; the production and sale of other commodities (notably coffee, copra, sesame, and peanuts) was encouraged, and railroads were built to Kigoma on Lake Tanganyika and to Moshi. In addition, many new Christian missions, which included rudimentary schools for the Africans, were established.

In 1954, Julius Nyerere and Oscar Kambona transformed the Tanganyika African Association (founded in 1929) into the more politically oriented Tanganyika African National Union (TANU). TANU easily won the general elections of 1958–60, and when Tanganyika became independent on Dec. 9, 1961, Nyerere became its first prime minister. In Dec., 1962, Tanganyika became a republic within the Commonwealth of Nations, and Nyerere was made president. On Apr. 26, 1964, shortly after a leftist revolution in newly independent Zanzibar, Tanganyika and Zanzibar merged; Nyerere became the new country’s first president. Abeid Amani Karume, the head of Zanzibar’s government and leader of its dominant Afro-Shirazi party (ASP), became Tanzania’s first vice president. Although formally united with the mainland, Zanzibar retained considerable independence in internal affairs. In Feb., 1967, Nyerere issued the Arusha Declaration, a major policy statement that called for egalitarianism, socialism, and self-reliance. It promised a decentralized government and a program of rural development called ujamaa (“pulling together”) that involved the creation of cooperative farm villages. Factories and plantations were nationalized, and major investments were made in primary schools and health care. While Nyerere put some of the declaration’s principles into practice, it was not clear if power in Tanzania was, in fact, being decentralized. TANU was the mainland’s sole legal political party and it was tightly controlled by Nyerere. In the early 1970s there was tension (and occasional border clashes) between Tanzania and Uganda, caused mainly by Nyerere’s continued support of Uganda’s ousted president, A. Milton Obote. However, in 1973, Nyerere and Gen. Idi Amin, Uganda’s new head of state, signed an agreement to end hostilities. Tanzania supported various movements against white-minority rule in S Africa, and several of these organizations had offices in Dar-es-Salaam. In 1977, TANU and Zanzibar’s ASP merged to form the Party of the Revolution (CCM). A new constitution was adopted the same year.