The details were revealed in the 2016 Global Slavery Index, a research report by the Walk Free Foundation, an initiative set up by Australian billionaire mining magnate and philanthropist Andrew Forrest in 2012 to draw attention to the issue.
Behind India was China (3.39 million), Pakistan (2.13 million), Bangladesh (1.53 million) and Uzbekistan (1.23 million).
The report said India had the highest number of people trapped in slavery at 18.35 million, while North Korea had the highest incidence (4.37 percent of the population) and the weakest government response.
The report suggested that there were 28 percent more slaves than estimated two years ago, a revision reached through better data collection and research methods.
Modern slavery refers to situations of exploitation that a person cannot leave because of threats, violence, coercion, abuse of power or deception.
They may be held in debt bondage on fishing boats, against their will as domestic servants or trapped in brothels.
Some 124 countries have criminalised human trafficking in line with the UN Trafficking Protocol and 96 have developed national action plans to coordinate the government response.
However, Forrest said more robust measures were needed.
“We call on governments of the top 10 economies of the world to enact laws, at least as strong as the UK Modern Slavery Act 2015, with a budget and capability to ensure organisations are held to account for modern slavery in their supply chains, and to empower independent oversight,” he said.
“I believe in the critical role of leaders in government, business and civil society. Through our responsible use of power, strength of conviction, determination and collective will, we all can lead the world to endslavery.”
As a percentage of the population, Uzbekistan (3.97 percent) and Cambodia (1.65 percent) trailed North Korea, which the study said was the only nation in the world that has not explicitly criminalised any form of modern slavery.
The report also tracked actions and responses to the problem, with governments at the forefront including the United States, Australia, and a host of European nations including Britain, Portugal and Norway.
Those with the weakest action included Iran, Hong Kong and China.
It cited Croatia, Brazil and the Philippines as countries to take positive steps since the last GlobalSlavery Index in 2014, while praising India for making significant progress in addressing the problem.
The firm reporting on modern slavery compiled information from 167 countries with 42,000 interviews in 53 languages to determine the prevalence of the issue and government responses.