The Republican Union (, UR) was a Spanish republican party founded in 1903 by Nicolás Salmerón.
It participated in the 1903, 1905 and 1907 general elections. Salmerón's decision in 1906 to join the Catalan Solidarity alliance would result in Alejandro Lerroux's split into the Radical Republican Party (PRR), as well as in a number of defections and in the decline of the party. It was mostly dissolved in 1910, being succeeded by the Republican Nationalist Federal Union, though some remnants lingered until 1920.