This is about the former political party in Spain; there was also a Republican Union party in Puerto Rico.

The Republican Union Party were a centrist political party in Spain led by Diego Martinez Barrio, formed as a split from the Radical Party. In 1936 led them into the Spanish Popular Front, a collection of left-wing political parties brought together for the purpose of contesting the 1936 election. The Popular Front won the election with many of their representatives in the Cortes coming from the Republican Union Party.