The Syrian Democratic Forces (SDF); Kurdish: Hêzên Sûriya Demokratîk‎ (HSD); Arabic: قوات سوريا الديمقراطية (قسد); Classical Syriac: ܚܝ̈ܠܘܬܐ ܕܣܘܪܝܐ ܕܝܡܩܪܛܝܬܐ‎, romanized: Ḥaylawotho d'Suriya Demoqraṭoyto) is an alliance in the Syrian Civil War composed primarily of Kurdish, Arab, and Assyrian/Syriac militias, as well as some smaller Armenian, Turkmen and Chechen forces. The SDF is militarily led by the People's Protection Units (YPG), a Kurdish militarily force. Founded in October 2015, the SDF states its mission as fighting to create a secular, democratic and federalised Syria. The updated December 2016 constitution of the Autonomous Administration of North and East Syria (AANES) names the SDF as its official defence force.

The primary opponents of the SDF are the various Islamist and Arab nationalist forces supported by Turkey, involved in the civil war, in particular the Islamic State of Iraq and the Levant (ISIL), the Syrian National Army (TFSA), al-Qaeda affiliates, the Turkish Armed Forces, and their allies. The SDF has focused primarily on the Islamic State, successfully driving them from important strategic areas, such as Al-Hawl, Shaddadi, Tishrin Dam, Manbij, al-Tabqah, Tabqa Dam, Baath Dam, and ISIL's former capital of Raqqa. In March 2019, the SDF announced the total territorial defeat of the Islamic State in Syria, with the SDF taking control of the last stronghold in Baghuz.

Since the Islamic State defeat, the SDF has increasingly been involved with combating the growing Turkish occupation of northern Syria.