Zamboanga Peninsula is both a peninsula and an administrative region in the Philippines. Designated as Region IX, the region consists of three provinces, namely, Zamboanga del Norte, Zamboanga del Sur, and Zamboanga Sibugay, and the chartered city of Zamboanga City at the south-eastern tip. The region was previously known as Western Mindanao before the enactment of Executive Order No. 36 on September 19, 2001.
After the United States annexed the Spanish East Indies in 1898, Zamboanga was briefly independent as the Republic of Zamboanga. It became a part of the Moro Province, which consisted of Mindanao and the Sulu Archipelago. The name and status of Moro province soon changed to the Department of Mindanao and Sulu on August 16, 1916, that caused Zamboanga to become a province.
On June 6, 1952, the province was divided into two provinces, Zamboanga del Norte and Zamboanga del Sur, while the chartered City of Zamboanga became an independent city.
Zamboanga Peninsula lies between the Moro Gulf, part of the Celebes Sea, and the Sulu Sea. Along the shores of the peninsula are numerous bays and islands. Its territory consists of the three Zamboanga provinces and Zamboanga City, as well as the Northern Mindanao province of Misamis Occidental. The peninsula is connected to the main part of Mindanao through an isthmus situated between Panguil Bay and Pagadian Bay. The boundary between the peninsula and the mainland is artificially marked by the border between the provinces of Zamboanga del Sur and Lanao del Norte.