Greater London covers an area of 609 square miles (1,579 km²). London is a port on the Thames, a navigable river. The river has had a major influence on the development of the city. London was founded on the north bank of the Thames and there was only a single bridge, London Bridge, for many centuries.

As a result, the main focus of the city was on the north side of the Thames. When more bridges were built in the 18th century, the city expanded in all directions as the mostly flat or gently rolling countryside around the Thames floodplain presented no obstacle to growth. There are some hills in London, examples being Parliament Hill and Primrose Hill, but these provided fine prospects of the city centre without significantly affecting the directions of the spread of the city and London is therefore roughly circular.

The Thames was once a much broader, shallower river than it is today. It has been extensively embanked, and many of its London tributaries now flow underground. The Thames is a tidal river, and London is vulnerable to flooding. The threat has increased over time due to a slow but continuous rise in high water level and the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound. The Thames Barrier was constructed across the Thames at Woolwich in the 1970s to deal with this threat, but in early-2005 it was suggested that a ten-mile-long barrier further downstream might be required to deal with the flood risk in the future.

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London has a temperate climate, with warm but seldom hot summers, cool but rarely severe winters, and regular but generally light precipitation throughout the year. Summer temperatures rarely rise much above 33°C (91°F), though higher temperatures have become more common recently. The highest temperature ever recorded in London was 38.1°C (100.6°F), measured at Kew Gardens during the European Heat Wave of 2003. Heavy snowfalls are almost unknown. In recent winters, snow has rarely settled to more than an inch (25 mm). London's average annual precipitation of less than 24 inches (600 mm) is lower than that of Rome or Sydney. London's large built-up area creates a microclimate, with heat stored by the city's buildings: sometimes temperatures are 5°C (9°F) warmer in the city than in the surrounding areas.