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.
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."
Xi said that China would launch a "South-South" assistance fund for developing countries with an initial investment of $2 billion.
The Chinese leader said that his country would also step up investment in least developed countries 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.
Xi said that China would 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.
The United Nations on Friday set a goal of ending extreme poverty by 2030, an effort that the global body says could require up to $5 trillion a year.