CO2 emissions are believed to be the driving force behind climate change.
The Paris deal is the world's first comprehensive climate agreement.
It will only come into force legally after it is ratified by at least 55 countries, which between them produce 55% of global carbon emissions.
"Ratification is yet to be done and India too is yet to do it. I announce that India will ratify the decision on October 2, the birth anniversary of Mahatma Gandhi," Mr Modi said in a speech at a national meeting of his ruling party in the southern state of Kerala.
Last December, countries agreed to cut emissions in a bid to keep the global average rise in temperatures below 2C.
The US and China - together responsible for 40% of the world's carbon emissions - both formally joined the Paris global climate agreement earlier this month.
India, one of the world's largest greenhouse gas emitters, will ratify the Paris global climate agreement pact next month, Prime Minister Narendra Modi has said.