The munitions – which are bombs that open in the air and release scores of smaller bomblets – are seen by the US as a way for Kyiv to use critically needed ammunition to help bolster its offensive and push through Russian front lines.
US leaders debated the thorny issue for months before President Joe Biden made the final decision last week.
The Pentagon says it will send cluster bombs with a reduced “dud rate”, meaning fewer of the smaller bomblets fail to explode.
The unexploded submunitions, which often litter battlefields and populated civilian areas long after a conflict ends, cause unintended deaths, particularly among curious children who pick them up to play with them
US officials said Washington will provide thousands of the rounds, but provided no specific numbers.
Lt Gen Douglas Sims, director of operations for the Joint Staff, told reporters on Thursday that “cluster munitions have indeed been delivered to Ukraine at this point”. But it was not clear if Ukrainian troops have used them yet.
Mr Biden described the decision to provide the projectiles as “very difficult”, citing their record of killing civilians.
More than 120 countries around the world – but not the US, Russia or Ukraine – have signed on to an international convention prohibiting the production of cluster munitions and discouraging their use.
Both Moscow and Kyiv have sent the munitions during the war, and Ukrainian regional officials have regularly accused Russian forces of using them on civilians.
Ukrainian President Volodymyr Zelenskyy on Wednesday thanked Mr Biden for the US military aid, and said cluster munitions would help in the fight against Russia.
The two met during Nato's summit in Lithuania, where western nations made fresh pledges of weapons and ammunition to fight Russia’s invasion.