The Social Democratic Union (Serbian: Социјалдемократска унија, romanized: Socijaldemokratska unija; abbr.

СДУ, SDU) was a minor social democratic and leftist political party in Serbia.

It was founded by former members of the Civic Alliance of Serbia, led by Žarko Korać, who opposed forming coalition with the right-wing Serbian Renewal Movement for the 1996 federal election.

[4][1] A year later, disenchanted members of the SDP, led by Žarko Korać, left and re-founded the SDU on 29 March 2003.

Spokesman of the SDP Ljiljana Nestorović stated that this was due to almost all local councils supporting co-president Slobodan Orlić, former leader of the SD, in the upcoming party congress which was to be held in less than 20 days.