Sydney is the most populous city in Australia, with a metropolitan area population of approximately 4.28 million (2006 estimate). It is the state capital of New South Wales, and was the site of the first British colony in Australia. It was established in 1788 at Sydney Cove by Arthur Phillip, leader of the First Fleet from Britain. A resident of the city is referred to as a Sydneysider.

Sydney is situated on Australia's south-east coast. The city is built around Port Jackson, which includes Sydney Harbour, leading to the city's nickname, "the Harbour City". It is noted for the Sydney Opera House and the Harbour Bridge, and its beaches. The metropolitan area is surrounded by national parks, and contains many bays, rivers and inlets. It is listed as a beta world city by the Loughborough University group's 1999 inventory and ranked 16th among global cities by Foreign Policy's 2008 Global Cities Index. The city has hosted international sporting events, including the 1938 British Empire Games, 2000 Summer Olympics and the final of the 2003 Rugby World Cup. The main airport serving Sydney is Sydney Airport.

Sydney is one of the most multicultural cities in the world, which reflects its role as a major destination for immigrants to Australia. According to the Mercer cost of living survey, Sydney is Australia’s most expensive city, and the 15th most expensive in the world.

Sydney's urban area is in a coastal basin, which is bordered by the Pacific Ocean to the East, the Blue Mountains to the West, the Hawkesbury River to the North and the Royal National Park to the South. It lies on a submergent coastline, where the ocean level has risen to flood deep river valleys (ria) carved in the hawkesbury sandstone. Port Jackson, better known as Sydney Harbour, is one such ria and is the largest natural harbour in the world. The Sydney area is not affected by significant earthquakes. The urban area has around 70 harbour and ocean beaches, including the famous Bondi Beach. Sydney's urban area covers 1,687 km2 (651 sq mi) as at 2001. The Sydney Statistical Division, used for census data, is the unofficial metropolitan area and covers 12,145 km2 (4,689 sq mi). This area includes the Central Coast, the Blue Mountains, and national parks and other unurbanised land.

Geographically, Sydney lies over two regions: the Cumberland Plain, a relatively flat region lying to the south and west of the harbour, and the Hornsby Plateau, a sandstone plateau lying mainly to the north of the harbour and dissected by steep valleys. The parts of the city with the oldest European development are located in the flat areas south of the harbour. The North Shore was slower to develop because of its hilly topography and lack of access across the harbour. The Sydney Harbour Bridge was opened in 1932 and linked the North Shore to the rest of the city.

Sydney has a temperate climate with warm summers and cool winters, and rainfall spread throughout the year. The weather is moderated by proximity to the ocean, and more extreme temperatures are recorded in the inland western suburbs. The warmest month is January, with an average air temperature range at Observatory Hill of 18.6-25.8 °C (65.5-78.4 °F) and an average of 14.6 days a year over 30 °C (86.0 °F). The maximum recorded temperature was 45.3 °C (113.5 °F) on 14 January 1939 at the end of a 4-day heat wave across Australia. The winter is mildly cool, with temperatures rarely dropping below 5 °C (41 °F) in coastal areas. The coldest month is July, with an average range of 8-16.2 °C (46.4-61.2 °F). The lowest recorded minimum at Observatory Hill was 2.1 °C (35.8 °F). Rainfall is fairly evenly divided between summer and winter, but is slightly higher during the first half of the year, when easterly winds dominate. The average annual rainfall, with moderate to low variability, is 1,217 mm (48 in), falling on an average 138 days a year. Snowfall was last reported in the Sydney City area in 1836. However, a July 2008 fall of graupel, or soft hail, mistaken by many for snow, has raised the possibility that the 1836 event was not snow, either.
Bondi Beach in Sydney's east. Sydney's warm weather in summer makes its beaches very popular.

The city is not affected by cyclones. The El Niño Southern Oscillation plays an important role in determining Sydney's weather patterns: drought and bushfire on the one hand, and storms and flooding on the other, associated with the opposite phases of the oscillation. Many areas of the city bordering bushland have experienced bushfires, notably in 1994 and 2001–02 — these tend to occur during the spring and summer. The city is also prone to severe hail storms and wind storms. One such storm was the 1999 hailstorm, which severely damaged Sydney's eastern and city suburbs. The storm produced massive hailstones of at least 9 cm (3.5 in) in diameter and resulting in insurance losses of around AUD $1.7 billion in less than five hours. The city is also prone to flash flooding from enormous amounts of rain caused by East Coast Lows (a low pressure depression which deepens off the state usually in winter and early spring which can bring significant damage by heavy rain, cyclonic winds and huge swells). The most notable event was the great Sydney flood which occurred on 6 August 1986 and dumped a record 327.6 mm (12.9 in) on the city in 24 hours. This caused major traffic problems and damage in many parts of the metropolitan area.

The Bureau of Meteorology has reported that 2002 through 2005 were the warmest summers in Sydney since records began in 1859. 2004 had an average daily maximum temperature of 23.39 °C, 2005 - 23.35 °C, 2002 - 22.91 °C and 2003 - 22.65 °C. The average daily maximum between 1859 and 2004 was 21.6 °C (70.9 °F). For the first nine months of 2006 the mean temperature was 18.41 °C (65.1 °F); the warmest year previously was 2004 with 18.51 °C (65.32 °F). Since November 2003, there have been only two months in which the average daily maximum was below average: March 2005 (about 1 °C below average) and June 2006 (0.7 °C below average).

The summer of 2007-08 proved to be one of the coolest on record. The Bureau of Meteorology reported that it was the coolest summer in 11 years, the wettest summer in six years, and one of only three summers in recorded history to lack a maximum temperature above 31 °C (88 °F).

Sydney hosts many different festivals and some of Australia's largest social and cultural events. These include the Sydney Festival, Australia's largest arts festival which is a celebration involving both indoor and free outdoor performances throughout January; the Biennale of Sydney, established in 1973; the Big Day Out, a travelling rock music festival which originated in Sydney; the Gay and Lesbian Mardi Gras along Oxford Street; the Sydney Film Festival and many other smaller film festivals such as the short film Tropfest and Flickerfest. Australia's premier prize for portraiture, the Archibald Prize is organised by the Art Gallery of New South Wales. The Sydney Royal Easter Show is held every year at Sydney Olympic Park, the final of Australian Idol takes place on the steps of the Opera House, and Australian Fashion Week takes place in April/May. Also, Sydney's New Years Eve and Australia Day celebrations are the largest in Australia.

In the year ending March 2008, Sydney received 2.7 million international visitors. The most well known attractions include the Sydney Opera House, and the Sydney Harbour Bridge. Other attractions include Royal Botanical Gardens, Luna Park, the beaches and Sydney Tower.

Sydney also has several popular museums such as, the Australian Museum (natural history and anthropology), the Powerhouse Museum (science, technology and design), the Art Gallery of New South Wales, the Museum of Contemporary Art and the Australian National Maritime Museum.