Xi added that his nation would settle disputes over contested waters with other powers in the region,
China has recently built artificial islands in the sea on top of previously submerged reefs, and has met with harsh criticism from the United States and key allies in the region, who accuse China of trying to expand its maritime territorial claims.
Late last month, a U.S. Navy warship made a close pass by built-upon reefs that China claims as its territory in contested waters
. Chinese ships and aircraft tracked the U.S. ship, and China's foreign ministry summoned the U.S. ambassador to the country to complain bitterly about the pass.
In his speech at the National University of Singapore on Saturday, Xi disputed that free passage has been a problem.
"The free passage of vessels and flights has never been a problem, and there never will be any problem, because China is the one that needs the free passage of vessels in the South China Sea the most," Xi said.
The sea has been Chinese territory since ancient times, and protecting it is a matter of China's territorial sovereignty, Xi said. It's a line of reasoning Beijing has often applied when justifying claims to maritime territory.
When speaking on any possible disputes in the South China Sea, Xi made mention of nations outside the region -- a possible reference to the United States -- but did not connect them directly with a resolution.
"We welcome countries outside the region to participate in the peace and development of Asia, and to make a positive impact," he said.
Xi is on a diplomatic trip in the region and met with with Taiwan's President Ma Ying-jeou in a historically significant meeting on Saturday.
It was the first time leaders from Taiwan and China have met since 1949, when the two split following China's civil war, which saw the Communist Party take control of the majority of China and Chiang Kai-shek's Nationalists flee to Taiwan.
China regards the island as a renegade province, but there are significant trade and cultural relations between the two.