The move by Russia, a longtime ally of President Bashar al-Assad's regime, is likely to anger Turkey, which considers the Kurdish People's Protection Units (YPG) to be a "terrorist" group.
"An agreement was signed between our units and Russian forces operating in Syria that will train us in modern military tactics," YPG spokesman Redur Xelil said.
"This is the first agreement of its kind, although we have had previous cooperation (with the Russians) in Aleppo city," he said.
The deal, which Xelil said was part "of the framework of the fight against terrorism," was signed on Sunday and came into force today.
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The YPG makes up the bulk of the Syrian Democratic Forces, a US-backed alliance of Kurdish and Arab fighters that has seized swathes of territory from the Islamic State group in northern Syria.
The SDF receives equipment, weapons and air support from the US-led coalition, and is backed by several hundred Western special operations forces in an advisory role.