La Unión de Isidoro Montes de Oca

 La Unión de Isidoro Montes de Oca  is a municipality in the Mexican state of Guerrero. The municipal seat lies at La Unión. The municipality covers an area of 1,142 km².

As of 2005, the municipality had a total population of 25,230.

The main towns are La Unión, population 3,079; Petacalco, population 2,671; Zacatula, population 1,417; Lagunillas, population 1,371; El Naranjito, population 1,167; and Surcua, population 1,046.

History
On October 26, 1614, the Dutch Captain Joris van Spilbergen - involved in his country's war with Spain - raided with an expedition of five ships this part of the coast of the then Spanish Mexico. At Zacatula he captured the pearl fishing ship San Francisco.