"This is where we engage in much of our commerce and buy many of our goods.
"This is where we can export more of our own products and create jobs back home in the process."
Obama, who will be spending three of his nine days in Asia in China, said Washington would approach the rising regional power "with a focus on our interests".
"And it is precisely for this reason that it is important to pursue pragmatic co-operation with China on issues of mutual concern, because no one nation can meet the challenges of the 21st century alone, and the US and China will both be better off when we are able to meet them together."
During his speech, Obama urged North Korea to return to stalled multilateral talks on its nuclear programme.
"We will not be cowed by threats, and we will continue to send a clear message through our actions," he said.
"And not just our words. North Korea's refusal to meet its international obligations will lead only to less security, not more."
Tokyo was the first stop in Obama's Asian tour, which also takes him to Singapore for an Asia-Pacific economic summit, to China for talks likely to feature climate change and trade imbalances, and to South Korea, where North Korea's nuclear ambitions will be in focus.
Yukio Hatoyama, Japan's recently elected prime minister, had promised to halt a Japanese naval mission supporting the US-led war in Afghanistan, review basing agreements for 47,000 US troops stationed in Japan, and explore the possibility of a new Asian trading block that would exclude the US.