Dating the origins of Swahili is not an easy task. It seems clear that the language was spoken at the coast during the 13th century. Some authors propose a much more ancient origin: in his work "Journey through the Erithraean Sea", a greek trader named Diogene who visited the East African coast in the year 110 A.D. told that the arab traders who regularly sailed the coast talked to the natives in their local language, which could represent the first historical reference to Swahili.

Though it was initially transcribed in arab writing, the origin of Swahili lies on the Bantu language, as shown by its grammar and syntax. Possibly, the absence of writing among the natives made that the first written texts in Swahili adopted the arab alphabet, which reached the coast first. Afterwards, the language was enriched with some arab terms, possibly when the Swahili population started reading the Koran. Shortly after, the Swahili succeeded as the common language for the coastal trade. Along the centuries, Swahili has adopted terms from other languages, such as Persian, Hindi, Portuguese, English and German. The portuguese influence during the domination of the coast by this European country shows up today in other features of Swahili culture, as in a bullfighting variant that is still practised in the island of Pemba.

Also in the 19th century, the European missionaries learnt Swahili in order to communicate with the natives. Johann Ludwig Krapf, the missionary who was the first European to describe Mount Kenya, authored the first translation of the Bible to Swahili, and also wrote the first Swahili dictionary and the first grammar. But the missionaries also contributed to spread the Swahili to the natives in the inner lands that spoke other languages. Afterwards, during the Colony days, the European rulers made an effort to standardise Swahili. The result of this attempt was the Kiswahili, or standard Swahili, taken from the Kiunguja dialect which was native to the town of Zanzibar (Unguja), which was the nucleus of the Swahili culture and cradle of the dialect considered the purest. The Kiswahili is today national language in Kenya and Tanzania.

The young urban people in Nairobi speak a peculiar dialect, a mixture of English, Swahili and some ethnic tongues, which is known as Sheng. Born in the city slums, Sheng has become a fashionable sign of modernity and cosmopolitanism for the Kenyan youth. Sheng uses mainly the Swahili grammar and syntax, but includes terms from other languages that can vary depending on the different environments.