Amid global tensions - over China's claims on the South China Sea and activist pressure on the US to address China's human rights record - one bright spot in US-China relations during this visit is likely to be agreements on climate change.
More nations will still need to accept the Paris agreement after the U.S. and China do for it to go into effect.
Both were key to getting an agreement in Paris a year ago.
China represents just over 20 percent of global emissions while the United States accounting for 17.9 percent, Russian Federation 7.5 percent and India 4.1 percent.
The G-20 summit opens Sunday in the eastern Chinese city of Hangzhou.
China's parliament on Saturday ratified the Paris agreement on climate change, the Xinhua state news agency said, which could help put the pact into force by as early as the end of the year.
Obama says there was "no shortage" of cynics who doubted an agreement would ever be reached.
Throughout his tenure, Obama has sought to check China's influence in Asia by shifting US military resources and diplomatic attention from the Middle East.
Saturday: China ratifies Paris climate pact, US tipped to follow
Before China's announcement, 23 countries had ratified or otherwise joined the agreement, representing just 1 percent of global emissions, according to the World Resources Institute.
Obama and Xi will begin the president's trip by spending most of Saturday together. He called for full implementation of the agreement to prevent or delay the worst effects of climate change.
Cyber issues, from concern over hacking and cyber espionage to emerging Chinese policies on information technology that foreign companies fear could limit their operations in the country, have also strained ties. The papers certified the USA and China have taken the necessary steps to join the Paris accord that set nation-by-nation targets for cutting carbon emissions.
The leaders met for more than four hours, first flanked by their top security and economic advisers, and later talking one-on-one as they took a night-time stroll around the picturesque West Lake, where Xi is hosting the G20 summit. Even the third step - formally participating in the deal - doesn't bring it into force in the USA or China.
The United States and China have formally joined the sweeping global climate change agreement reached in Paris a year ago.
U.S. President Barack Obama on Saturday pressed his Chinese counterpart Xi Jinping on territorial disputes in the South China Sea, urging Beijing to uphold its legal obligations and stressing the United States' commitments to its regional allies. The document certifies that the countries have taken all necessary domestic steps needed to join the agreement.
The two countries shall enhance people-to-people exchanges to deepen their traditional friendship, and strengthen coordination on global and regional affairs to contribute to a new type of worldwide relations that value cooperation and mutual benefit, Xi said.