The small craft, travelling at "high speed" after dark in international waters, was initially spotted by US aerial surveillance, the British embassy in the United Arab Emirates said.
The boat, travelling south from Iran, was intercepted by forces from British frigate HMS Lancaster before it could navigate back to Iranian territorial waters on February 23.
The weapons discovered on board included medium-range ballistic missile components as well Iranian versions of Russia's Kornet anti-tank missile, a statement said.
The seizure took place along a route historically used to smuggle weapons to Yemen, according to the Bahrain-based United States Fifth Fleet.
"This is the seventh illegal weapon or drug interdiction in the last three months and yet another example of Iran's increasing malign maritime activity across the region," said US Vice-Admiral Brad Cooper.
In January, the US Navy said it had seized more than 2,000 assault rifles smuggled on a fishing boat along a maritime route from Iran to Yemen.
The Iran-backed Huthi rebels took control of the Yemeni capital Sanaa in 2014, prompting a Saudi-led coalition to intervene the following year.
Since then, a grinding war has killed hundreds of thousands and pushed the impoverished nation to the brink of famine.
Fighting has largely been on hold since a UN-brokered ceasefire took effect in April last year, even after the agreement expired in October.
The UN Security Council expanded an arms embargo on the Huthis in February 2022.
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