The campaign, initiated by the World Wide Fund for Nature, involves millions of people around the world switching their lights off at 8.30pm, in their time zone, for one hour.
Earth Hour's Australian manager Anna Rose says some of Australia's tourism icons are involved, particularly in Sydney.
"Earth Hour is a symbolic action that highlights the need to tackle climate change," she said.
"We've got a lot of buildings showing their support - the Harbour Bridge is going out, the Opera House is going out.
"But the most important thing is the families that we're expecting will turn off their lights and use the opportunity to have a conversation to reflect on climate change and what it is doing to Australia."
Cities around the world taking part in Earth Hour
Around the world other iconic buildings are also taking part in the campaign.
In Paris, the lights of the Eiffel tower will be switched off, while in London hundreds of landmarks are involved with lights being turned off in places that include Buckingham Palace, Tower Bridge, Trafalgar Square, St Paul's Cathedral and Wembley Stadium.
Brighton Pier, Cardiff Castle and Edinburgh Castle are also involved in Earth Hour.
This is the critical decade to get Australia's carbon pollution down and increase renewable energy uptake if we're going to save the reef.
Anna Rose, Earth Hour Australia
In Jordan, a candlelit march will be held along one of the oldest streets in Jabal Amman and the lights of Al Karak Castle will be turned off.
Gwanghwamun Square, in the South Korean capital of Seoul, will be plunged into darkness for Earth Hour.
In the United States, landmarks switching off include the Golden Gate Bridge in San Francisco, Times Square and the Empire State building in New York, and the Space Needle in Seattle.
Even some of the hotels and casinos on the famous Las Vegas strip are turning off their gaudy displays.
'Time running out' for Great Barrier Reef
The Great Barrier Reef is the focus of the Australian Earth Hour campaign this year.
Almost 4,000 candles will spell "Lights out for the Reef" at Parliament House in Canberra.
One of the supporters of the campaign is Dr Selina Ward, a coral reef biologist who lectures at the University of Queensland and is a past president of the Australian Coral Reef Society.
Dr Ward says the Great Barrier Reef has been valued by Aboriginal and Torres Strait Islander people for more than 60,000 years and currently supports more than 63,000 livelihoods, including people working in the tourism industry and the fishing industry.
But she says the reef is one of the most vulnerable places on earth to climate change and the impact is heartbreaking.
These include the bleaching of coral, increasing ocean and sand temperatures, more extreme storm damage to the reef and rising sea levels.
"Our reef is running out of time, but those of us alive today can be the ones to help save it," she said.
Director of the Global Change Institute Ove Hoegh-Guldberg agrees.
He has spent 25 years researching the Great Barrier Reef and says local tour operators are worried about the future.
"The Great Barrier Reef, like coral reefs everywhere, is at a turning point," Professor Hoegh-Guldberg said.
"If we don't increase our commitment to solve the burgeoning stress from local and global sources, the reef will disappear.
"This is not a hunch or alarmist rhetoric by green activists. It is the conclusion of the world's most qualified coral reef experts."
Earth Hour 'a feel-good event'
Author Bjorn Lomborg, president of the Copenhagen Consensus Centre, describes Earth Hour as "an ineffective feel-good event".
He says it does little for the climate in terms of reducing CO2 emissions and distracts from real problems.
These include 3 billion people in developing countries who burn traditional fuels indoors to keep warm and cook food because they do not have electricity.
He says this creates noxious fumes which are linked to 4.3 million deaths each year, as reported by the World Health Organisation.
Ms Rose says a broader event - Earth Hour Action - is also being launched so people can pledge to take other action on climate change beyond tonight's event.
Earth Hour momentum growing in Australia
Earth Hour is now in its eighth year and Ms Rose says the momentum is growing, particularly in Australia.
"It is a powerful symbol and we've been really overwhelmed by the amount of support this year for the Great Barrier Reef and for Earth Hour," she said.
"We have the biggest number of events ever registered in Australia this year for Earth Hour. Well over 1,000, which shows that Australians still do want action on climate change.
"We're a passionate country when it comes to the reef and people want to make sure we do protect it.
"The good thing about Earth Hour is it is taken on by thousands of schools across the country, also businesses, councils, community groups, sporting groups and book clubs and that really resonates with the Earth Hour message that no-one can do everything but everyone can do something," said Ms Rose.