India, which currently ranks as the second most populous country with 1.3 billion inhabitants, will surpass China's 1.4 billion citizens by 2024, the UN said in a report.
The world's total population is expected to hit 9.8 billion by 2050 despite universal lower fertility rates,the report added.
The world's population is now at least 7.6 billion, up from 7.4 billion last year, spurred by the relatively high levels of fertility in developing countries -- despite an overall drop in the number of children people have around the globe, the report World Population Prospects: The 2017 Revision revealed on Wednesday.
The concentration of global population growth is in the poorest countries, presenting a challenge as the world seeks to implement the 2030 Sustainable Development Agenda, which is aimed to end poverty and preserve the planet, according to the report.
"With roughly 83 million people being added to the world's population every year, the upward trend in population size is expected to continue, even assuming that fertility levels will continue to decline," said the UN Department of Economic and Social Affairs.
The growth is expected to come, in part, from the 47 least developed countries, where the fertility rate is around 4.3 births per woman, and whose population is expected to reach 1.9 billion people in 2050 from the current estimate of 1 billion.
In addition, the birth rates in African countries are likely to "at least double" by 2050, said the report.
That trend comes despite lower fertility rates in nearly all regions of the world, including in Africa, where rates fell from 5.1 births per woman up to 2005 to 4.7 births in the five years following.
In contrast, the birth rates in Europe are up to 1.6 births per woman, up from 1.4 births in 2000-2005.
The lower fertility rates are resulting in an ageing population, with the number of people aged 60 or over expected to more than double by 2050 and triple by 2100, from the current 962 million to 3.1 billion.
Africa, which has the youngest age distribution of any region, is projected to experience a rapid ageing of its population, the report noted.
By 2050, the third most populous country will be Nigeria, which currently ranks seventh, and which is poised to replace the US.