Mindanao is the second largest island in the country with 94,630 square kilometers. The island is mountainous, and is home to Mount Apo, the highest mountain in the country. Mindanao is surrounded by seas: the Sulu Sea in the west, the Philippine Sea in the east, and the Celebes Sea in the south.
The island group of Mindanao is an arbitrary grouping of islands in the southern Philippines which encompasses six administrative regions. These regions are further subdivided into 25 provinces, of which only four are not on Mindanao island itself. The island group includes the Sulu Archipelago to the southwest, which consists of the major islands of Basilan, Jolo, and Tawi-Tawi, plus outlying islands in other areas such as Camiguin, Dinagat, Siargao, Samal, and the Sarangani Islands.

The following are six regions of the island of Mindanao:

 Zamboanga Peninsula (Region IX)

 Northern Mindanao (Region X)

 Davao Region (Region XI)

 SOCCSKSARGEN (Region XII)

 Caraga (Region XIII)

 Autonomous Region in Muslim Mindanao (ARMM)

Western Mindanao now called Zamboanga Peninsula (Region IX) - is located in the landform of the same name. It consists of the provinces of Zamboanga del Norte, Zamboanga del Sur, Zamboanga Sibugay, and two cities—Zamboanga City and Isabela City—which are independent of any province. Isabela City is the only territory not on Mindanao island itself and is a part of Basilan. The region's new administrative capital is Pagadian City and the whole region used to be a single province named Zamboanga.

Northern Mindanao (Region X) - consists of the provinces of Bukidnon, Camiguin, Lanao del Norte, Misamis Occidental, Misamis Oriental, Cagayan de Oro City, and Iligan City. The province of Camiguin is also an island just of the northern coast. The administrative center and capital of the region is Cagayan de Oro City.

Southern Mindanao now called Davao Region (Region XI) - is located in the southeastern portion of Mindanao. The region is divided into the provinces of Davao Oriental, Davao, Davao del Sur, and Compostela Valley; plus Davao City. The region encloses the Davao Gulf to the south and includes the island of Samal in the gulf, and the Sarangani Islands further to the south. Davao City is the region's administrative center.

Central Mindanao now called SOCCSKSARGEN (Region XII) - is located in the south-central portion of the island. It consists of the provinces of Cotabato, Sarangani, South Cotabato (which was used to be part of Region XI), and Sultan Kudarat, plus Cotabato City. The names of the provinces together with General Santos City spell the name of the region which is an acronym. Cotabato City, which is surrounded by, but is not a part of Maguindanao province, was the region's former administrative center. Koronadal City, in South Cotabato, is the new administrative center of the newly formed region.

Caraga (Region XIII) - is located in the Northwestern part of Mindanao. Its provinces are Agusan del Norte, Agusan del Sur, Surigao del Norte, and Surigao del Sur. The administrative center is Butuan City in Agusan del Norte. The region also covers the outlying islands of Surigao del Norte such as Dinagat Island, Siargao Island, and Bucas Grande Island.

The Autonomous Region in Muslim Mindanao (ARMM) - is a special region which covers the territories predominantly populated by Muslims. ARMM has its own government unlike almost all the other regions in the country. It consists of almost the whole of the Sulu Archipelago (Isabela City while remaining a part of Basilan is in the Zamboanga Peninsula region) and two provinces in the mainland. The provinces located in the Sulu Archipelago are Basilan, Sulu, and Tawi-Tawi. Basilan and Tawi-Tawi are themselves the main islands of their respective provinces, while the main island of Sulu is Jolo Island. The mainland provinces are Lanao del Sur and Maguindanao. Cotabato City, while not a part of the ARMM, is the region's administrative center.

Source: Wilkpidia