"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.
Beijing's sweeping claims also overlap with those of the Philippines, Malaysia, Brunei and Taiwan.
Trump's comments come ahead of a state visit on Sunday afternoon by Chinese President Xi Jinping who will meet with top communist leaders.
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.
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.
Trump heads to Manila later Sunday for the final stop on his marathon Asia tour that has mainly focused on building regional consensus against North Korea's nuclear ambitions and espousing his 'America First' vision of global trade.