US Coast Guard cutter Monomoy rescued six Iranian mariners after their vessel broke down in the Gulf.
One of the six had suffered burns and is receiving treatment, Pentagon spokesman George Little told reporters.
The US Navy rescued 13 Iranian fishermen last Thursday after they were captured by Somali pirates.
Both incidents come at a time of heightened tensions between Washington and Tehran.
In the latest incident, the six Iranians were some 50 miles (90km) south-east of the Iraqi port of Umm Qasr when they got into trouble early on Tuesday, Mr Little said.
The crew used flares and flashlights to hail the passing Monomoy. The vessel's master indicated "that the engine room was flooding and [the ship was] not seaworthy", Mr Little added.
The Pentagon said the Iranian mariners would be repatriated, although it has not yet been determined how.
Last Thursday, forces assigned to the John C Stennis aircraft carrier group - on its way from the Gulf to Afghanistan - received a distress call from the Al Molai fishing vessel.
The ship had been boarded by pirates in the Arabian Sea more than a month ago, and its crew had been kept in "harsh conditions", the Pentagon said at the time.
A US Navy spokesman said the force went out of its way to "treat the fishing crew with kindness and respect" and provided them with fuel and food for their return journey home.
Washington says its forces routinely rescue sailors in distress - regardless of nationality - but wanted to highlight the two recent incidents in light of the current tensions between the two countries.
Tehran has not commented on Tuesday's incident, but described last Thursday's rescue as a "humanitarian gesture".
However, days earlier Tehran had warned the carrier not to return to the region once it left for Afghanistan.
Faced with the prospect of fresh sanctions over its nuclear programme, Iran has threatened to block the transport of oil through the Strait of Hormuz.
Washington has rejected Iran's warning to keep its forces out of Gulf waters, and has warned it will resist any attempt by Tehran to close the Strait.
A US ship has picked up Iranians in trouble at sea, in the second such rescue in less than a week, the Pentagon has said.