The seizure, which unfolded in the Arabian Sea on March 28, was the third of its kind in recent weeks, military officials say. Iran has been supporting Houthi rebels in Yemen in their proxy war against a Saudi-led coalition backed by the United States. Like Iran, the Houthis are a Shia-led group.
The arms shipment appears to mark the latest provocative action from the Islamic republic. On Friday, President Obama said Iran was obeying the "letter" of its landmark nuclear agreement with the West, but not the "spirit" of it.
The Navy said the shipment included 1,500 AK-47s, 200 rocket-propelled grenade launchers and 21 .50-caliber machine guns.
After the U.S. seized the weapons stash from the dhow, a traditional sailing vessel, the Navy let the crew go. A U.S. official told Fox News current rules do not allow western naval forces to seize the crew in addition to illicit cargo. "You have to find a country willing to prosecute," the official said.
A defense official reached by Fox News would not reveal the nationality of the dhow's crew.
Last month, Iran announced that it tested missiles marked with the phrase "Israel must be wiped out," in violation of a U.N. Security Council resolution tied to the recent nuclear deal. The resolution forbids Iran from working on its ballistic missile program for eight years and bans sales of its conventional weapons.
In January, Iran captured 10 U.S. Navy sailors in the Persian Gulf, on the same day President Obama delivered his State of the Union address. Iran released the sailors one day later.
Last year, naval forces from Iran's Revolutionary Guard seized a Marshall Islands-flagged cargo ship and held it for weeks to settle a business dispute. Iranian vessels later surrounded a U.S.-flagged vessel but did not detain it.
Soon afterwards, U.S. warships escorted all U.S. and British flagged cargo ships and tankers transiting the Strait of Hormuz, a key Mideast oil passage.
In late December, Iran's military fired rockets near the USS Harry S. Truman aircraft carrier, a move the U.S. called "unnecessarily provocative and unsafe."
Fox News' Lucas Tomlinson and The Associated Press contributed to this report.