A small crowd cheered and applauded as the skyline went dark. The Opera House was lit in a deep shade of green to symbolise renewable energy.
The annual event is organised by WWF, the global environment campaign.
It encourages cities and individuals to switch off non-essential lights for an hour at 20:30 local time.
Critics have dismissed the campaign, now into its seventh year, as a gimmick of little or no practical benefit.
This year, Russian President Vladimir Putin is taking part for the first time.
The Kremlin due to switch off its lights at 16:30 GMT along with 90 other landmark buildings in Moscow.
Sydney cut its lights at 09:30 GMT.
"It's really exciting," local resident Jessica Bellamy told AFP news agency.
"It's been a very inspiring night because it's all about hope and change."
One person wrote on Twitter: "Sydney Harbour bridge is in darkness for Earth Hour. You can even see the stars!"
Australia's biggest city, Sydney, has switched off its lights for an hour as the Earth Day climate change protest gets under way across the globe.