Nearly 36 million people worldwide, or 0.5% of the world's population, live as slaves, a survey by anti-slavery campaign group Walk Free says.
The group's Global Slavery Index says India has the most slaves overall and Mauritania has the highest percentage.
The total is 20% higher than for 2013 because of better methodology.
The report defines slaves as people subject to forced labour, debt bondage, trafficking, sexual exploitation for money and forced or servile marriage.
It uses slavery in a modern sense of the term, rather than as a reference to the broadly outlawed traditional practice where people were held in bondage and treated as another person's property.
The Global Slavery Index's estimate is higher than other attempts to quantify modern slavery. In 2012, the International Labour Organisation estimated that almost 21 million people were victims of forced labour.
Call for action
Walk Free says it found evidence of slavery in all 167 countries it surveyed.
The report says Africa and Asia face the biggest challenges in eradicating slavery, while the practice is least prevalent in Europe.
According to the report, more than 14 million people live as slaves in India. Next in the index comes China, with more than 3 million slaves, followed by Pakistan and Uzbekistan.
Russia is ranked fifth. The country's economy is said to rely on enslaved migrant workers in the construction and agricultural sectors.
Mauritania meanwhile has the highest number of slaves as a proportion of the population, at 4%. Many people in the African country inherit their slave status from their ancestors.
The report calls for much wider international cooperation on slavery. It wants governments to increase penalties for trafficking and to put pressure on businesses to clamp down on the use of slaves in their supply chains.
The Global Slavery Index was first published last year. The rise in the overall figure from 2013 was attributed by the report's authors to better data and methodology, rather than to an exponential rise in the numbers enslaved.
The UK is listed in the top 10 countries taking action against slavery. Its parliament is due to debate a law on Monday that will allow tougher sentences to be imposed for human trafficking.
Top five countries with the highest proportion of slaves
Mauritania - 4%
Uzbekistan - 3.97%
Haiti - 2.3%
Qatar - 1.36%
India - 1.14%
Figures from Global Slavery Index, 2014