The 1903 Southern African Customs Union Agreement was a multilateral treaty between the British colonies and protectorates in Southern Africa that created a customs union between the territories.
The Governor of the Cape Colony, who was the High Commissioner for Southern Africa, also signed on behalf of Basutoland and the Bechuanaland Protectorate.
The Agreement established a customs union area with free trade amongst the parties.
The success of the customs union encouraged some British residents to seek political unification, which ultimately resulted in the creation of the Union of South Africa in 1910.
(Basutoland, Southern Rhodesia, and Swaziland did not join the political union.)