It is a 2nd class municipality in the province of Pangasinan, Philippines.
The indigenous people of Calasiao are descended from the Austronesian-speaking people who settled in the Malay archipelago at least 5,000 years ago. Calasiao was settled by a Pangasinan speaking people whose language belongs to the Malayo-Polynesian branch of the Austronesian languages family.
In 1571, the Spanish conquest of Pangasinan began. The Spanish conquistadors were accompanied by Roman Catholic missionaries who introduced Roman Catholicism to the indigenous peoples of Pangasinan. Around 1592, the small settlement of Calasiao was united with the other neighboring small settlements of Nalsian and Dinalaoan to form the new town of Calasiao. The much larger settlement of Gabon and other settlements also became part of the new town of Calasiao. The town of Calasiao became part of the Pangasinan encomendia of Labaya, designated as belonging to the King of Spain, Juan Ximenez del Pino, and a son of Alonso Hernandez de Sandoval for whom tributes were collected.