The US coast guard gave its Vietnamese counterpart a high endurance cutter in a ceremony in Honolulu on Thursday, the embassy in Hanoi said.
The gift comes several days after the US delivered six patrol boats to Vietnam's coast guard to strengthen its "maritime law enforcement" and humanitarian response operations.
"This cutter provides a concrete and significant symbol of the US-Vietnamese comprehensive partnership," US Coast Guard Rear Adm. Michael J. Haycock was quoted as saying during the handover in Hawaii.
The move, the "first freedom of navigation" exercise under US President Donald Trump, sparked ire from China, who claims nearly all of the sea.
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Taiwan, Vietnam and several other Southeast Asian nations also claim parts of waterway, which is believed to sit atop vast oil and gas deposits.
But Hanoi has found itself increasingly alone in challenging China's island building campaign, especially as the Philippines warms up to the regional superpower.
US President Donald Trump's rise to the White House has also cast uncertainty over American foreign policy in the region.
Security and trade ties between the former wartime foes have deepened since Washington restored diplomatic relations with Hanoi in 1995.