Sydney ( SID-nee) is the capital city of the state of New South Wales, and the most populous city in Australia. Located on Australia's east coast, the metropolis surrounds Sydney Harbour and extends about 70 km (43.5 mi) towards the Blue Mountains to the west, Hawkesbury to the north, and the Royal National Park and Macarthur to the south and south-west. Greater Sydney consists of 658 suburbs, spread across 33 local government areas. Residents of the city are colloquially known as "Sydneysiders". The estimated population in June 2022 was 5,297,089; the city is home to approximately 66% of the state's population. The city's nicknames include the "Emerald City" and the "Harbour City".

Sydney is well endowed with open spaces and has many natural areas. Many of these exist even within the compact city centre. These include the Chinese Garden of Friendship and Hyde Park (which is named after London's Hyde Park). The metropolitan area contains several national parks, including the Royal National Park, the second oldest national park in the world (after Yellowstone National Park), which occupies an area of 132 square kilometres (51 sq mi; 13,200 ha; 33,000 acres). Completing Sydney's wide array of green spaces, the leader is the Royal Botanical Gardens, with its large amount of green spaces, lush plants and colourful flowers.

Although Sydney developed organically after the arrival of the First Fleet, the city parks and open spaces were a part of early town planning to provide relief from the bustle and monotony of the city streets. Hyde Park is the oldest park in the city.

Transport in Sydney is provided by an extensive network of public transport operating modes including metro, train, bus, ferry and light rail, as well as an expansive network of roadways, cycleways and airports. According to the 2006 census, in terms of travel to work or study Sydney has the highest rate of public transport usage among the Australian capital cities of 26.3% with more than 80% of weekday trips to/from Central Sydney being made by public transport. According to the New South Wales State Plan, the state has Australia's largest public transport system. The public transport network is regulated by Transport for NSW.

As climate change, greenhouse gas emissions and pollution have become a major issue for Australia, Sydney has in the past been criticised for its lack of focus on reducing pollution and emissions and maintaining water quality. The release of the Metropolitan Air Quality Scheme (MAQS) led to a broader understanding of the causation of pollution in Sydney, allowing the government to form appropriate responses. The 2019–20 Australian bushfire season significantly impacted outer Sydney and dramatically reduced air quality, leading to a smoky haze that lingered for days. The air quality was 11 times the hazardous level in some days, worse than New Delhi's; it was compared to "smoking 32 cigarettes" by Brian Oliver, a respiratory diseases scientist at the University of Technology Sydney.

Australian cities are some of the most car-dependent cities in the world, especially by world city standards, although Sydney's is the lowest of Australia's major cities at 66%. Sydney also has the highest usage of public transport in an Australian city, at 27%–comparable with New York City, Shanghai and Berlin. Despite its high ranking for an Australian city, Sydney has a low level of mass-transit services, with a historically low-density layout and significant urban sprawl, thus increasing the likelihood of car dependency. Strategies have been implemented to reduce private vehicle pollution by encouraging mass and public transit, initiating the development of high density housing and introducing a fleet of 10 new electric cars, the largest order of the pollution-free vehicle in Australia. Electric cars do not produce carbon monoxide and nitrous oxide, which contribute to climate change. Cycling trips have increased by 113% across Sydney's inner-city since March 2010, with about 2,000 bikes passing through top peak-hour intersections on an average weekday. Transport developments in the north-west and east of the city have been designed to encourage use of the expanding public transportation system.

The City of Sydney became the first council in Australia to achieve formal certification as carbon-neutral in 2008. The city has reduced its 2007 carbon emissions by 6% and since 2006 has reduced carbon emissions from city buildings by up to 20%. The Sustainable Sydney 2030 program presented a guide to reducing energy in homes and offices by 30%. Reductions in energy consumption have slashed energy bills by $30 million a year. Solar panels have been established on many CBD buildings to minimise carbon pollution by around 3,000 tonnes a year.

The city also has an "urban forest growth strategy", in which it aims to regular increase the tree coverage in the city by frequently planting trees with strong leaf density and vegetation to provide cleaner air and create moisture during hot weather, thus lowering city temperatures. Sydney has also become a leader in the development of green office buildings and enforcing the requirement of all building proposals to be energy-efficient. The One Central Park development, completed in 2013, is an example of this implementation.