At that point, both nations will number 1.45 billion people each. Subsequently India's population will continue to grow until the middle of the century, while China's slowly declines.
The UN also estimates that the current global population of 7.2 billion will reach 9.6 billion by 2050.
That is a faster rate of growth than previously estimated.
The population growth will be mainly in developing countries, particularly in Africa, the UN says.
The world's 49 least developed countries are projected to double in size from around 900 million people in 2013 to 1.8 billion in 2050, whereas the population of developed regions will remain largely unchanged.
The UN said the reason for the increase in its projection is largely new information on fertility levels in certain high birth rate countries.
