The cathedrals in Birmingham, Gloucester, Lincoln, Manchester and Salisbury also joined the Earth Hour 2017 initiative, organised by the World Wide Fund for Nature (WWF).
Colin Butfield, director of campaigns at WWF, said on Saturday: "Today, hundreds of millions of people will be showing global unity on climate change during Earth Hour. Climate change is impacting us here and now.
"We are seeing it across the globe, from the Great Barrier Reef suffering mass bleaching for an unprecedented second year in a row, to more severe weather in Britain."
Well done NSW & VIC! Your turn QLD, SA, NT & WA! #EH17 #EarthHour #EarthHour2017 #SydneyHarbourBridge #jointhefuture pic.twitter.com/UiGbCLXJtu
— WWF_Australia (@WWF_Australia) March 25, 2017
Among the churches taking part was St Peter's Church in Carmarthen, where the floodlights were turned off and a Celtic worship-themed service was held by candlelight.
Speaking with Premier beforehand, assistant curate Sulin Milne urged other churches to get involved, saying: "You could do something as simple as just turning off your floodlights so the church suddenly disappears from the scene and maybe people will think, 'hey, why has that happened?'"
The Houses of Parliament joined the Eiffel Tower in Paris, the Empire State Building in New York and the Kremlin in Moscow in marking Earth Hour between 8.30pm and 9.30pm.
Colin Butfield added: "Following the tragic events in London earlier this week, we are inspired and grateful to hear that the Houses of Parliament and Big Ben will be switching off their lights to show support for global action on climate change."
"Earth Hour is the world's biggest climate change event. All you have to do is switch off your lights for one hour to join hundreds of millions around the world to send a clear signal that we must act and we must act now."
Other UK landmarks taking part included Edinburgh Castle, the Senedd Building in Cardiff, Buckingham Palace, Blackpool Tower and Brighton Pier.
Sulin Milne from St Peter's added: "We hope that it will, first of all, raise awareness of environmental concerns but also make people realise that the Church cares about this."
Listen to Premier's Alex Williams speaking with Sulin Milne: