The event aims to raise global consciousness about practical actions that people can take to prevent global warming.
Organisers are encouraging people to pledge online to do something small but sustainable in their own lives to improve the planet's health – such as switching to compact fluorescent light bulbs or reducing the use of pesticides and other toxic chemicals.
"Millions of people doing small, individual acts can add up to real change," said Chad Chitwood, a spokesman for the umbrella group coordinating efforts.
Earth Day was born in the US, but 192 countries now celebrate it.
More than 20 million people took part in the first Earth Day, and helped inspire ground-breaking US legislation including the Clean Air Act, Clean Water Act, the Endangered Species Act and other groundbreaking laws.
For one day, online search engine Google has replaced its usual logo with an animated doodle of a natural vista, including pandas that sneeze and salmon that leap when you move your mouse over them.
(EurActiv with Reuters.)