It is the capital city of the Department of its same name. It is the largest city in the department with a population of almost 26,739 inhabitants. The town, previously known as Puerto San Carlos, was given the name La Unión on 13 July 1824, following independence from Spain and the formation of the United Provinces of Central America.
On January 16, 2005 the government of El Salvador started building a new port, in order to replace the old one. It will be able to hold more than double the cargo held by the current principal port of El Salvador at Acajutla. The new port was to be finished by November 2012. It will help to increase the growth in this side of the country. One half of the growth is located at the country's capital San Salvador, but a lot of commerce is also transferred to San Miguel, being also the nearest largest city.