The US has sent roughly 1.1 million bullets seized from Iran last year to Ukraine, its military has said.
The US Central Command (Centcom), which oversees operations in the Middle East, says the rounds were confiscated from a ship bound for Yemen in December, reports BBC.
Ukraine's Western allies recently warned that their production lines were struggling to keep up with the rate at which Ukraine was using ammunition.
Centcom says the Iranian rounds were given to Ukraine on Monday.
The munitions were originally seized by US naval forces from a stateless ship named MARWAN 1 on 9 December, it said.
The US government gained ownership of them in July through a process known as civil forfeiture, by which an asset can be seized if its owner is thought to be involved in criminal activity.
In this case, the claim was brought against Iran's Islamic Revolutionary Guard Corps, a branch of the Iranian armed forces tasked with preserving the country's government.
Iran backs the Houthi rebels in Yemen's ongoing civil war, but arms transfers to the group are barred under a 2015 resolution by the UN Security Council.
The civil war in Yemen began in 2014 when the Houthis took control of the capital Sanaa and removed the country's government.
The ousted government remains the internationally recognised government of Yemen and is backed by a Saudi-led coalition of countries in the region as well as the US and the UK.
Since the second half of last year, Iran has also repeatedly been accused of supplying Russia with arms, most notably drones, for use in the war in Ukraine.