Rosario, originally a sitio and later a barrio of Santo Tomas, La Union, became a municipality in 1869 through the efforts of local leaders who petitioned for its independence. Early settlers came from nearby towns such as Sto. Tomas, Agoo, Magsingal, Tubao, Bacnotan, and San Fernando, spreading across various barrios. The town’s name is believed to have originated from either the abundance of wild roses (“rosas del rio”) along its rivers or from a mountain range resembling a rosary. As the southernmost town of La Union and the first from the south into the Ilocos region, Rosario is fondly known as the “Gateway to Ilocandia,” offering visitors a scenic entrance into the heart of Ilocano land with its lush forests, flowing rivers, and rolling hills.