The transport system has helped reduce pollution levels in the city by 630,000 tonnes a year, a UN release said.
If not for the Metro, the 1.8 million people who use it daily would have travelled by cars, buses or motorbikes, adding to pollution, it added.
It will now get $9.5m (Â£6.1m) in carbon credits annually for seven years.
And as the number of passengers increase, so will this figure.
Carbon credits are generated by a UN-run scheme called the Clean Development Mechanism (CDM).
The mechanism gives firms in developing countries financial incentives to cut greenhouse gas emissions.
“The United Nations body administering the clean development mechanism under the Kyoto Protocol has certified that Delhi Metro has reduced emissions,” the UN statement said.
“No other Metro in the world could get the carbon credit for the above because of the very stringent requirement to provide conclusive documentary proof of reduction in emissions,” it added.
Every passenger who uses the Metro instead of cars or buses helps reduce greenhouse emissions by approximately 100gm of carbon-dioxide for every trip of 10km (6 miles) and that helps in reducing global warming, the UN said.
Delhi’s hi-tech metro system was launched in 2002. Parts of the network are underground while some sections use elevated tracks.
The system, which covers some of the city’s most congested streets, is seen as the answer to Delhi’s traffic chaos and has helped in lowering air pollution levels.