Singapore consists of 63 islands, including mainland Singapore. There are two man-made connections to Johor, Malaysia - Johor-Singapore Causeway in the north, and Tuas Second Link in the west. Jurong Island, Pulau Tekong, Pulau Ubin and Sentosa are the largest of Singapore's many smaller islands. The highest natural point of Singapore is Bukit Timah Hill at 166m.

The south of Singapore, around the mouth of the Singapore River and what is now the Downtown Core, used to be the only concentrated urban area, while the rest of the land was either undeveloped tropical rainforest or used for agriculture. Since the 1960s, the government has constructed new residential towns in outlying areas, resulting in an entirely built-up urban landscape. The Urban Redevelopment Authority was established on 1 April 1974, responsible for urban planning.

Singapore has on-going land reclamation projects with earth obtained from its own hills, the seabed, and neighboring countries. As a result, Singapore's land area grew from 581.5 km2 (224.5 sq mi) in the 1960s to 704 km2 (271.8 sq mi) today, and may grow by another 100 km² (38.6 sq mi) by 2030. The projects sometimes involve some of the smaller islands being merged together through land reclamation in order to form larger, more functional islands, such as in the case of Jurong Island.

Under the Köppen climate classification system, Singapore has a tropical rainforest climate with no distinctive seasons. Its climate is characterized by uniform temperature and pressure, high humidity, and abundant rainfall. Temperatures range from 22 °C to 34 °C (72° to 93 °F). On average, the relative humidity is around 90% in the morning and 60% in the afternoon. During prolonged heavy rain, relative humidity often reaches 100%. The lowest and highest temperatures recorded in its maritime history are 19.4 °C (66.9 °F) and 35.8 °C (96.4 °F) respectively.

June and July are the hottest months, while November and December make up the wetter monsoon season. From August to October, there is often haze, sometimes severe enough to prompt public health warnings, due to bush fires in neighboring Indonesia. Singapore does not observe daylight saving time or a summer time zone change. The length of the day is nearly constant year round due to the country's location near the equator.

About 23% of Singapore's land area consists of forest and nature reserves. Urbanization has eliminated many areas of former primary rainforest, with the only remaining area of primary rainforest being Bukit Timah Nature Reserve. A variety of parks are maintained with human intervention, such as the Singapore Botanic Gardens.