Southern Nigeria was a British protectorate in the coastal areas of modern-day Nigeria formed in 1900 from the union of the Niger Coast Protectorate with territories chartered by the Royal Niger Company below Lokoja on the Niger River.

The Lagos colony was later added in 1906, and the territory was officially renamed the Colony and Protectorate of Southern Nigeria. In 1914, Southern Nigeria was joined with Northern Nigeria Protectorate to form the single colony of Nigeria.

The unification was done for economic reasons rather than political—Northern Nigeria Protectorate had a budget deficit; and the colonial administration sought to use the budget surpluses in Southern Nigeria to offset this deficit.
Sir Frederick Lugard, who took office as governor of both protectorates in 1912, was responsible for overseeing the unification, and he became the first governor of the newly united territory. Lugard established several central institutions to anchor the evolving unified structure.

A Central Secretariat was instituted at Lagos, which was the seat of government, and the Nigerian Council (later the Legislative Council), was founded to provide a forum for representatives drawn from the provinces. Certain services were integrated across the Northern and Southern Provinces because of their national significance—military, treasury, audit, posts and telegraphs, railways, survey, medical services, judicial and legal departments—and brought under the control of the Central Secretariat in Lago