Isabela National High School is a public educational institution established in the City of Ilagan, Isabela, Philippines. It was founded in 1904, making it the oldest public high school in the Cagayan Valley region. It was named Isabela High School before it was converted into a national high school in 1982. It is the only educational institution offering the Open High School Program in the entire Cagayan Valley region. As of 2014, the school is administered by Denizon P. Domingo, the head of the Schools Division of Ilagan. The school's principal is Samuel P. Lazam.

''Since the school's founding in June, 1904, many of those who passed through its portals became illustrous leaders and outstanding citizens, occupyng highly influential positions until the national level. Many have also found themselves well established in foreign soils and performing creditably well as expected of every alumnus. Graduates of the school numbering about a hundred thousand since its first graduation in 1921 had certainly made clear footprints in time and productively contributing to socio-economic development of the country.'' - Dr. Dominador Z. Cabasal of class 1948, A Brief History of the Isabela National High School