UNEP Executive Director Erik Solheim said the move by the two of the world's largest economies was significant, Xinhua news agency reported.
"This announcement is hugely important. The leadership of China and the US is crucial to taking the Paris Agreement forward. They bring significant additional momentum to keeping global warming under two degrees," Solheim said in a statement issued in Nairobi.
Presidents of China and the United States handed over their countries' instruments of joining the Paris Agreement separately to UN Secretary-General Ban Ki-moon on Saturday in China's Hangzhou.
The Paris Agreement, which was sealed in December 2015 after two weeks of intense negotiations, needs to be ratified by 55 countries, representing 55 per cent of global emissions, in order to come into effect.
Experts said both countries joining the Paris Agreement could help bring the accord into affect by the end of 2016.
"By putting the well-being of our planet at the top of the agenda, the two largest economies in the world are also showing that our economic future is low-carbon and green," Solheim said.
He said the fight against climate change remains difficult and urgent, but having heavy-hitters like China and the United States on "your side is extremely heartening".
