[6] The name "Agoo" is believed to have originated from a native tree locally known as "aroo" in Ilocano and "agoho" in Tagalog (Casuarina equisetifolia), commonly referred to as the whistling pine.

[8] Historically, the tree grew abundantly along the riverbanks and the forested western coastline of Agoo, playing a significant role in the town's natural landscape.

[1] The history of the settlement now known as Agoo dates back even further, with both documentary and artifactual evidence supporting the assertion that it was a major port of call for foreign traders before it was formally established by the Spaniards.

[2] In its early history, Agoo served as a trading port in the northern region of ancient Pangasinan (modern day-Pangasinan and La Union provinces).

It was already a coastal maritime trading center for Northwestern Luzon[10] due to its naturally shaped coastline, which provided an excellent harbor for foreign vessels entering Lingayen Gulf.

[10] Evidence of trade between Agoo and China has been excavated in the form of porcelain and pottery pieces unearthed at the site of the Catholic church during its renovation, which are now kept in the Museo de Iloko.

These Japanese would leave when the port of Agoo was later closed, but not without first teaching the natives their methods of fish culture, rice cultivation, deerskin tanning, duck breeding, and weapons manufacturing.

Agoo encompassed a vast land area that spanned the modern-day towns of Rosario, Santo Tomas, Tubao, Pugo, Aringay, Caba, Bauang, and the place called "Atuley" or present-day San Juan.

[2] Father Aquilino Garcia constructed a church, and by the end of the 15th century, the image of Nuestra Señora de Caridad (Our Lady of Charity) was installed in it.

Led by General Manuel Tinio y Bondoc, a close ally of Emilio Aguinaldo, the local population engaged in battles against Spanish forces in Northern Luzon.

These guerrilla forces employed tactics such as ambushes, sabotage, and coordinated attacks on American supply lines to disrupt enemy operations.

The U.S. military government implemented strategies to suppress any remaining resistance, including strict curfews, intelligence-gathering operations, and the establishment of garrisons.

Public hospitals were also built, improving access to healthcare for residents, while infrastructure development, such as roads and bridges, facilitated economic growth.

These developments brought notable changes to Agoo’s social, educational, and political environment, shaping the town’s transition into the modern era under American occupation.

Capturing the three towns which were connected by a high quality road, but protected on one side by the ocean and the other by the sea, meant that the Japanese forces were easily able to establish a secure beachhead.

[18] The Japanese' 47th Infantry Regiment under the command of Col Isamu Yanagi, accompanied by the 4th Tank Regiment and supported by a flotilla of the Imperial Japanese Navy led by Vice Admiral Kenzaburo Hara (consisting of the light cruiser Natori , destroyers Fumizuki, Nagatsuki, Satsuki, Minazuki, Harukaze, Hatakaze, three minesweepers, six anti-submarine craft and six transports) was supposed to land on the beaches of Agoo beginning 5:00 A.M. on December 22, 1941, having left Takao on Taiwan the evening of December 18.

[21][22][23] : "43"  La Union residents mostly did not speak up about the economic stresses they were experiencing, but when Marcos declared martial law in 1972, Agoo was included with the rest of the country.

[31] The Agoo municipal building collapsed completely, killing numerous citizens who were inside because they were in line to pay in time for the national income tax deadline for that quarter.

Nieva reported seeing the Virgin Mary, popularly known as Our Lady of Agoo atop a Guava tree, a statue weeping with blood became highly sensationalized.

On June 11, 2014, then-representative Eufranio Eriguel filed House Bill 4644 to establish the first city in the second district by merging the municipalities of Agoo and Aringay.

[44] On August 16, 2016, former Congressman Eriguel was included by Philippine President Rodrigo Duterte as one of the local government officials and legislators allegedly involved in illegal drug trade[45] in his "I am sorry for my country" speech.

[49] In 2021, the Philippines' Department of Environment and Natural Resources (DENR) established[50] the Agoo Eco-Park in a 10,774.68 hectare area (of which 3% is land in Barangay Sta Rita West, while the rest is part of the marine ecosystem) has already been designated designated as a Protected Landscape and Seascape as part of the Agoo–Damortis Protected Landscape and Seascape (ADLPS) in 2018, and 2021 marked the beginning of systematic reforestation efforts under the national greening program (NGP) which the national government first launched in 2011.

[50] Philippine Native trees found in the survey included the Mangrove pagatpat (Sonneratia alba), Ayangile (Acacia confusa), Aroo (Casuarina equisetifolia), Talisay (Terminalia catappa L.) and Bakawan lalaki (Rhizophora apiculata) and native undergrowth species included Bagaswa (Ipomoea pes-caprae L.), Kawad-kawaran (Cynodon dactylon) and Putok-putok (Ruellia tuberosa).

[53] Agoo is a coastal municipality located on a narrow plain between the foothills of the Cordillera Central and the Lingayen Gulf, also known as the South China Sea.

Agoo is one of the southern municipalities of the province and is bordered on the north by Aringay, on the east by Tubao, which lies further up the foothills of the Cordilleras, and on the south by Sto.

The San Manuel soil is a dark grayish-brown sandy loam with medium compactness and a pH ranging from highly acidic to slightly alkaline.

The Maligaya soil is a dark grayish-brown clay loam with coarse fragments of soft, powdery red and black concretions, characterized by medium compactness and a neutral to slightly alkaline pH.

Agoo is predominantly Roman Catholic, with significant numbers of people adhering to other Christian denominations, such as Iglesia ni Cristo, Protestantism, Aglipayan, Pentecostal and Jehovah’s Witnesses.

Aquaculture is also a major industry in the town, with residents engaged in the cultivation of tilapia and bangus (milkfish), which are important for both local consumption and trade.

These businesses provide essential goods and services to the community and contribute to Agoo’s economic development, with a growing emphasis on entrepreneurship and local commerce.