NEW DELHI—India will ratify the 2015 global climate agreement early next month, Prime Minister Narendra Modi said Sunday, bringing the deal a significant step closer to taking effect this year.

The accord, struck in Paris by 195 countries, laid out a path to curb global warming and, for the first time, required developing countries to take action to lower the trajectory of their emissions growth. ‎

But it can take effect only once 55 countries, representing 55% of the world’s greenhouse-gas emissions, join it. So far, 60 countries have done so, including the largest emitters, the U.S. and China, fulfilling the first condition.

These countries together account for 47.76% of global greenhouse gas emissions. Once India ratifies, that figure will go up to nearly 52%, according to the World Resources Institute, an environmental think tank in Washington.

Mr. Modi said at an event of his Bharatiya Janata Party that India would ratify on Oct. 2. A further 13 countries have also committed to ratify the deal by the end of the year, all but ensuring that the agreement will take effect in 2016.