Cagayan (/kɑːɡəˈjɑːn/ kah-gə-YAHN, [kagɐˈjan]), officially the Province of Cagayan (Ilocano: Probinsia ti Cagayan; Ibanag: Provinsiya na Cagayan; Itawit: Provinsiya ya Cagayan; Isnag: Provinsia nga Cagayan; Ivatan: Provinsiya nu Cagayan; Gaddang: Provinsia na Cagayan; Filipino: Lalawigan ng Cagayan), is a province in the Philippines located in the Cagayan Valley region, covering the northeastern tip of Luzon. Its capital is Tuguegarao, the largest city of that province as well as the regional center of Region 2 (Cagayan Valley). It is about 431 kilometres (268 mi) northwest of Manila, and includes the Babuyan Islands to the north. The province borders Ilocos Norte and Apayao to the west, and Kalinga and Isabela to the south.

Cagayan was one of the early provinces that existed during the Spanish colonial period. Called La Provincia de Cagayan, its borders essentially covered the entire Cagayan Valley, which included the present provinces of Isabela, Quirino, Nueva Vizcaya, Batanes and portions of Kalinga, Apayao, and Aurora. The former capital was Nueva Segovia, which also served as the seat of the Diocese of Nueva Segovia. Today, only 9,295.75 square kilometres (3,589.11 sq mi) remain of the former vastness of the province. The entire region, however, is still referred to as Cagayan Valley.

A folk legend holds that the name was originally derived from the tagay, a plant that grows abundantly in the northern part of the province. The term Catagayan, "the place where the tagay grows" was shortened to Cagayan. Linguists, however, hold that cagayan comes from an ancient, lost word that means "river". Variations of this word—karayan, kayan, kahayan, kayayan, kagayan and kalayan—all mean river, referring to Cagayan River. It is said to have also shared etymology with Cagayan de Oro in Mindanao with the similar reference, also called Cagayan River.

Cagayan has a prehistoric civilization with rich and diverse culture. According to archaeologists, the earliest man in the Philippines probably lived in Cagayan thousands^[specify] of years ago.^{[citation needed]}

In the classical era, Gattaran and Lal-lo was the home of hunter-gatherers who specialized in hunting mollusks. These hunter-gatherers have stockpiled their leftover mollusk shells in numerous sites in Gattaran and Lal-lo, until eventually, the shells formed into the largest stock of shell-midden sites in the entire Philippines.

The Atta or Negritos were the first people in valley. They were later moved to the uplands or variably assimilated by the Austronesians, from whom the Ibanags, Itawes, Yogads, Gaddangs, Irayas and Malawegs descended - who actually came from one ethnicity. These are the people found by the Spaniards in the different villages along the rivers all over Cagayan. The Spaniards rightly judged that these various villagers came from a single racial stock and decided to make the Ibanag language the lingua franca, both civilly and ecclesiastically for the entire people of Cagayan which they called collectively as the Cagayanes which later was transliterated to become Cagayanos.

Cagayan was a major site for the Maritime Jade Road, one of the most extensive sea-based trade networks of a single geological material in the prehistoric world, operating for 3,000 years from 2000 BCE to 1000 CE.

Even before the Spaniards came to Cagayan, the Cagayanos have already made contact with various civilizations like the Chinese, Japanese and even Indians, as evidenced by various artifacts and even the presence of minor to moderate foreign linguistic elements in the languages of the natives.