In a strongly worded response, China stressed the South China Sea as one of its "core interests", putting the issue up there with Taiwan, Tibet and Xinjiang in terms of its importance to Beijing.
China warned its rival claimants and neighbors not to be emboldened by U.S. support - a line it has repeated ever since.
An arbitration court hearing the dispute between China and the Philippines over the South China Sea is set to hand down its ruling on in the Dutch city of The Hague on Tuesday. Legal experts expect at least some substantive findings to go against China, which has vowed to ignore the ruling.
China's military, which is ramping up its presence in the South China Sea as part of a major modernisation programme, is also watching the election closely.
"Hillary is very fierce when it comes to China," a Chinese official close to the military establishment told Reuters.
While the Chinese government has been largely quiet about the U.S. election, state media has not been so restrained, with one paper even equating Trump to Hitler.
In May, China's official Xinhua news agency noted Trump's more isolationist campaign compared to Clinton's, who it described as an "old foreign policy hand" and important backer of the Asia-Pacific "pivot" that China considers a threat.
"As far as she's concerned, being tough on foreign policy is perhaps the best way to show America's so-called 'leadership'," it said in a commentary.
Laura Rosenberger, a Clinton campaign foreign policy adviser who worked with her in the State Department, told Reuters Clinton would remain tough on the South China Sea issue.
"She believes that we need to be very strong in terms of standing up to many of the actions the Chinese have taken," Rosenberger said.
"She believes in the principles of freedom of navigation in international waters, that commerce on the high seas in incredibly important to the United States, and that these are really very direct interests that we need to continue to stand up for."
Trump adviser Peter Navarro, an economist at University of California Irvine and the author of the book, "Crouching Tiger: What China's Militarism Means for the World," said a Trump presidency would lead to respect.
"The central difference between a Trump administration and the current administration or a Clinton administration is respect. The leaders of Russia, the leaders of China will respect Mr. Trump, will respect America because we will be strong economically, militarily and politically."
Trump may actually find some sympathy in China, even if he is seen as an unknown quantity.
"Who is Trump? We don't really know. We do know he hates Muslims though - and that will be well received in some circles here," said the Chinese official with ties to the military, pointing to what China views as its
war on terror in its Muslim-populated far western region