Xi's announcement, made in his first-ever address to the United Nations, follows long-standing criticism from the United States and other developed countries that China has not taken responsibilities in line with its aspirations for a greater global role.
The speech is the latest high-profile move for Xi on a trip that has included a White House state visit and will see him chair a UN forum on women's rights on Sunday.
Addressing a UN summit on development, Xi said China would act "by putting justice before interests and joining other countries in a concerted effort to realize the post-2015 agenda."
The Chinese leader said that his country would also step up investment in UN-defined least developed countries -- which are mostly in Africa -- by at least $12 billion by 2030.
China has been an increasingly active investor around the world, although it has generally focused on seeking resources rather than broader humanitarian goals.
In January, Xi promised USD 250 billion in investment over 10 years in Latin America, which the United States has long considered its sphere of influence.
Xi said that China would also relieve debts owed by least developed countries this year. He did not provide a figure on the debts or say which countries would be affected.
China's economy has soared over the past 15 years to become the world's second largest after the United States, although concerns have been rising in recent months over the country's financial markets and long-term growth.
China has increasingly taken an assertive role in the world, especially in myriad disputes with its neighbors, but retains little stake in the World Bank and International Monetary Fund.
With a reform plan for the Washington-based institutions blocked by US Republican lawmakers, China has set up its own lender, the Asian Infrastructure Investment Bank.
"It is important to improve global economic governance, increase the representation and voice of developing countries and give all countries equal right to participating in international rule-making," Xi said.
