Setul, officially the Kingdom of Setul Mambang Segara (Malay: Kerajaan Setul Mambang Segara; Jawi: نڬري ستول ممبڠ سڬارا‎; Thai: เมืองสตูล; RTGS: Mueang Satun) was a Malay kingdom founded in the northern coast of the Malay Peninsula. The state was established in 1808 in the wake of the partition between the rulers of the royal house of Kedah. The partition witnessed the territory being seceded to the cadet branch of the royal family. The sovereignty of the kingdom effectively ended in 1916, following the dissolution by the Siamese government. Its borders were largely inherited to its successive province, the present-day Satun, Thailand.

Setul derives its name from Buah Setul, a local name for the cottonfruit tree native to the area, while its honorific title—Mambang Segara—is a Malay variant of the Sea Deity, potentially tied from ancient Malay mysticism due to its location off the western seaboard of the Malay Peninsula. An interpretation of Mambang Segara can also be seen on its present-day provincial seal.

The state was also colloquially referred to as Setoi, a Kedahan Malay cognate of the territory. In the Thai language, the state was known as Satun (Thai: สตูล). The Thai pronunciation was later adopted to the present-day name of the province.

Once located in the heartland of Kedah Kingdom, Setul rose to prominence following the death of Abdullah Mukarram Shah, the 20th sultan of Kedah, in 1797. The demise of the monarch resulted in the crowning of Ahmad Tajuddin Halim Shah II as the new sultan. The election was endorsed and recognised by the Rattanakosin Kingdom of Siam, which perceived Kedah as their vassal. The appointment, however, rapidly erupted into a severe secession crisis in the royal house after it was contested by the crown prince, Tunku Bisnu. Aiding to reconcile the two rivalling parties, the Siamese appointed Tunku Bisnu as the ruler of Setul, signifying the birth of Kedah into two separate realms.

Tunku Bisnu spent most of his reign in Kedah in Setul, with local affairs being mainly administrated by his aide, Dato' Wan Abdullah. Nonetheless, as recorded by Syair Sultan Maulana, it was narrated that Tunku Bisnu was a capable ruler that led the Kedahan troops during the war against the Burmese Konbaung dynasty forces in Salang (present-day Phuket, Thailand).

By 1833, the royal courts of Kedah entered another crisis between Sultan Ahmad Tajuddin Halim Shah II and Tunku Embun (Tunku Yaakub), the crown prince. The crisis was predominantly derived from the sultan's refusal to appoint the crown prince to be the king of Kayang (Perlis) and Setul. This resulted in Tunku Embun having to request military assistance from the governor of Ligor to overthrow the Kedahan government, falsely claiming that the sultan had allied with the British. The armed conflict was known locally as Perang Musuh Berbisik (War of the whispering enemies).

During the war of 1833, the Kedahan militia led by Dato Wan Mad Ali (Dato Setia Sengkara) launched an attack against the Siamese in Terang, Ayer Kelubi and Pulau Panjang. Following the fall of Kedah, Dato Wan Mad Ali was forced to move to Langkawi along with an exodus of 100 Malay families from Setul.

The coup d'état led by Tunku Yaakob proved to be successful, as the sultan was forced to retreat to the British-backed Penang and later exiled in Malacca. Nonetheless, Tunku Yaakob's request to be appointed as the sultan of Kedah was refused. He and his son, Tunku Sulaiwan, were later brutally executed by the order of the Siamese king after he was found guilty of spreading malicious news of the previous sultan.