Provisional results from India's latest census put the population at 1.21 billion, a rise of 181 million in a decade.
India now has more people than Indonesia, the United States, Brazil, Pakistan and Bangladesh combined.
The increase alone is equivalent to more than the population of Pakistan or Brazil, but the growth rate is nonetheless the lowest in nearly a century.
Literacy rates also grew, but the census commissioner says a further widening in the gap between the numbers of male and female children is deeply worrying.
He says it shows that decades of social policies, including a drive to eradicate female infanticide, are not working.
India's child gender ratio has declined to 914 females for every 1000 males, the biggest imbalance since independence in 1947.
Sex-selective abortion based on ultrasound scanning is illegal in India but mothers come under huge pressure to produce male heirs who are seen as wage-earners and future family leaders.