China claims almost all of the resource-rich waterway, through which trillions of dollars in trade passes annually, with competing claims from the Philippines, Brunei, Malaysia, Taiwan and Vietnam.
The Senate and House of Representatives also declared that his separately elected vice presidential running mate, Sara Duterte, had won by a wide margin.
With an initial count almost complete, Marcos Jr has secured over 56 per cent of the vote and more than double the tally of his nearest rival, liberal Leni Robredo.
Ferdinand Marcos placed the Philippines under martial rule in 1972, a year before his term was to expire.
Here are five things to know about relations between the United States and one of its oldest allies in Asia.