Ilagan, officially known as the City of Ilagan (Ibanag: Siudad nat Ilagan; Ilocano: Siudad ti Ilagan; Tagalog: Lungsod ng Ilagan), is a 1st class component city and capital of the province of Isabela, Philippines. According to the 2020 census, it has a population of 158,218 people making it the most populous city in the province. It holds the title as the most number of voters in the province with 98,565 electorates. Ilagan was a first class municipality before it became a city.The city is located on the central portion of the province of Isabela. It is bounded by nine municipalities: on the north by the municipalities of Divilacan, Tumauini, and Delfin Albano; on the west by the municipality of Quirino; on the east by the municipalities of Divilacan, Palanan, and the Pacific Ocean; and on the south by the municipalities of Gamu, Naguilian, Benito Soliven and San Mariano. Ilagan is approximately 96 kilometres (60 mi) from Tuguegarao and 397 kilometres (247 mi) from Metro Manila (linked by a national highway via Santa Fe, Nueva Vizcaya).

With a total land area of 116,626 hectares (288,190 acres), it is the largest city in the island of Luzon and the fourth largest city in land area in the Philippines, after Davao City, Puerto Princesa, and Zamboanga City.

A municipality (Tagalog: bayan/munisipalidad; Hiligaynon: banwa; Cebuano: lungsod/munisipalidad; Pangasinan: baley; Kapampangan: balen/balayan; Central Bikol: banwaan; Waray: bungto/munisipyo; Ilocano: ili) is a local government unit (LGU) in the Philippines. It is distinct from city, which is a different category of local government unit. Provinces of the Philippines are divided into cities and municipalities, which in turn, are divided into barangays (formerly barrios) – villages. As of 7 September 2019, there are 1,488 municipalities across the country.A municipality is the official term for, and the official local equivalent of, a town, the latter being its archaic term and in all of its literal local translations including Filipino. Both terms are interchangeable.

A municipal district is a now-defunct local government unit; previously certain areas were created first as municipal districts before they were converted into municipalities.