"If I can help mediate or arbitrate, please let me know... I am a very good mediator," the dealmaking American leader told Vietnamese President Tran Dai Quang during an official state visit today.
Vietnam has long courted support from Washington in its dispute with China over the precious waterway, where Beijing has built military installations and artificial islands.
Beijing's sweeping claims also overlap with those of the Philippines, Malaysia, Brunei and Taiwan.
Tensions over the sea spiked earlier this year when Vietnam suspended an oil exploration project in an area of the waterway also claimed by Beijing, reportedly over pressure from its powerhouse communist neighbour.
More From This Section
In 2014, China moved an oil rig into waters off Vietnam's shore, sparking violent protests in several Vietnamese cities.
Trump is in Hanoi for a brisk visit that could touch on boosting military support for America's former wartime foe.
Talks will also focus on promoting trade between the cosy allies, though Vietnam's gaping trade deficit worth $32 million could be a sticking point.