One of the most widely accepted theories suggests that the name derives from the phrase “rosas del río” ("roses of the river"), referring to the picturesque landscape observed by the Spanish explorers upon their arrival.

This landscape was characterized by narrow valleys, wild animals, birds, rivers, brooks, dense forests, and a scenic riverscape.

The Marukos chastised them for engaging in leisure activities on a holy day and placed a curse upon them, causing them to lose their sense of direction.

[7] In the highlands of town, in the foothills of the Cordillera Central and along the Bued River, the Cordillerans (Igorot), specifically the Ibaloi people, established their communities.

[8]The inhabitants of these settlements were actively involved in trade and barter, engaging with neighboring groups such as the Ilocanos and Tagalogs, as well as with foreign merchants from Japan and China.

These goods were traded for luxury items, tools, and other valuable commodities brought by foreign merchants, reflecting a vibrant and interconnected economy.

By 1578, the Franciscans had established a permanent settlement in the area, constructing a visita (a small mission chapel) made of bamboo.

The present-day towns of Rosario, Santo Tomas, Tubao, Pugo, and Aringay were originally part of Agoo's territorial jurisdiction.

[10] Don Lorenzo de los Reyes was appointed as the first gobernadorcillo of Santo Tomas, responsible for local governance under the Spanish colonial system.

[10] During the 19th century, Rosario remained a barrio of Santo Tomas and was inhabited by a mixture of Igorot rancherías, Pangasinenses, and Ilocano settlers.

[10] Increasing population pressures, combined with the harsh conditions under Spanish rule, led to a significant migration from Ylocos provinces from Magsingal, Vigan, Candon, and nearby towns such as Agoo, Santo Tomas, Tubao, and San Fernando.

[10][11] On October 29, 1849, Governor-General Narciso Zaldua Clavería issued a decree creating the province of La Union by merging eight towns from Pangasinan, three from Ilocos Sur, and 40–45 rancherías, including those in Rosario.

This new provincial structure was formalized on March 2, 1850, through a Superior Decreto signed by Governor-General Antonio María Blanco and confirmed on April 18, 1854, by a royal decree (Real Orden) from Queen Isabella II of Spain.

[10] Despite this administrative progress, the local Catholic bishop expressed concerns about assigning an assistant priest to Rosario, citing the diminished stipends caused by the town’s separation from Santo Tomas.

[10] Consequently, Rosario became La Union’s 13th civil town but remained spiritually dependent on its former matriz (mother parish) in Santo Tomas.

[10] The majority of the population maintained strong ties to their ancestral lands and primarily engaged in agriculture and commerce, cultivating crops such as rice, tobacco, corn, sweet potatoes, and sibucao (a type of wood used for dye).

[10] During the initial phase of the Philippine Revolution in early 1897, the people of Rosario did not immediately take up arms but instead expressed sympathy for the uprising.

Under the leadership of General Manuel Tinio y Bondoc, a close ally of President Emilio Aguinaldo, the locals engaged in significant resistance efforts in Northern Luzon.

[10] Following the surrender of General Manuel Tinio's brigade to American forces in Sinait, Ilocos Sur, on May 1, 1901, the Philippine-American War in La Union came to an end.

However, on October 25, 1907, Governor-General James Francis Smith issued a mandate reinstating Rosario as a regular municipality, restoring the original boundaries of the 12 towns established during the Spanish period.

[10] In addition to education, the American government set up public hospitals to combat cholera epidemic, with Rosario establishing a one-man health board.

[13] The Japanese occupation of the town ended after the Battle of Bacsil Ridge in San Fernando, marking a significant turning point in the liberation of La Union.

[13] Rosario is a coastal town located at the southernmost point of La Union, situated on a narrow plain between the foothills of the Cordillera Central mountain range and the Lingayen Gulf or South China Sea.

[3] The town has seen a steady rise, with an average annual population growth rate (PGR) of 1.77%, meaning approximately 18 people are added per year for every 1,000 residents.

[18] The majority of Rosario's population belongs to the Ilocano ethnolinguistic group, making Iloco the predominant language spoken in the municipality.

Additionally, smaller groups follow Islam and the Union Espiritista Cristiana de Filipinas Inc., reflecting the religious diversity within the municipality.

The Bued Communal Irrigation System (CIS II) received international funding support in 1998 from the Agrarian Reform Infrastructure Support Project (ARISP) through the government of Japan, which directly benefited the PUSUNANGTATAY Agrarian Reform Community (ARC) encompassing six clustered barangays: Puzon, Udiao, Subusub, Nangcamotian, Tabtabungao, and Tay-ac.

[14] The Rosario Public Market is located in Barangay Subusub along McArthur Highway, occupying a total area of 9,781 square meters.

[14] Rosario's tourism potential is significant, with its diverse landscapes of mountains, rolling hills, valleys, and beaches providing ample opportunities for development.

[14] Eco-tourism sites in the municipality include Rocapor Farm, where visitors can enjoy organic fruit and vegetable picking, nature appreciation, picnics, and animal feeding.