Kathiri (Arabic: كثيري), officially the Kathiri State of Seiyun or the Kathiri State in Hadhramaut (Arabic: سلطنة الكثيري حضرموت سيئون) was a sultanate in the southern Arabian Peninsula, in what is now Yemen. It was located in the inland portion of Hadhramaut with its capital at Seiyun (Say'un). The sultanate entered into treaty relations with the British in the late 19th century and became a part of the Aden Protectorate. Kathiri declined to join the Federation of South Arabia but remained under British protection as part of the Protectorate of South Arabia. In 1967, the sultanate was abolished and Kathiri became part of newly independent South Yemen which united with North Yemen in 1990 to become the Republic of Yemen.