The Union is the number 3 canton of the province of Cartago, in Costa Rica, being the smallest of the eight that compose it, but the most densely populated. The canton has an area of 44.83 km² and its head is the city of Tres Ríos. It was declared officially by Legislative Decree No.167 of December 7,1848 as the number three canton of the province of Cartago, with a parish district. Protected by the Political Constitution of November 30,1848, which in its article 8, establishes for the first time the denominations of canton province and parish district, for which La Union is one of the oldest cantons of Costa Rica.
It must be taken into account, that the Canton is La Unión, and not as it is sometimes incorrectly called Tres Ríos, without considering that Tres Ríos is the first district of the Canton of La Unión, and that its decree was a cantonal declaration and not of its foundation. Its first founders, these natives, began their arrival in the territory of La Union, between 1717, and by 1725 there was a more abundant population. According to the 2011 National Census, the population of the canton was 99,399 inhabitants, of which 9.7% were born abroad. The same census highlights that there were 26,979 occupied homes, of which, 69.7% were in good condition and there were problems of overcrowding in 4.7% of the houses.97.0% of its inhabitants lived in urban areas. Among other data, the literacy level of the canton is 98.6%, with an average schooling of 9.5 years. The same census details that the economically active population is distributed as follows: Primary Sector: 1.8%. Secondary Sector: 20.1%. Tertiary Sector: 78.1%.