As Colombo possesses a natural harbour, it was known to Indian, Greek, Persian, Roman, Arab, and Chinese traders over 2,000 years ago. Traveller Ibn Battuta who visited the island in the 14th century, referred to it as Kalanpu. Arabs, whose prime interests were trade, began to settle in Colombo around the 8th century AD mostly because the port helped their business by the way of controlling much of the trade between the Sinhalese kingdoms and the outside world. Their descendants now comprise the local Sri Lankan Moor community.

Colombo is located on the west coast of the island and adjacent to the Greater Colombo area which includes Sri Jayawardenepura Kotte, the legislative capital of Sri Lanka and Dehiwala-Mount Lavinia.

Colombo features a tropical monsoon climate.

Colombo's climate is fairly temperate all throughout the year.

From March to April the average high temperature is around 31 °C (87.8 °F). The only major change in the Colombo weather occurs during the monsoon seasons from May to August and October to January. This is the time of year where heavy rains can be expected. Colombo sees little relative diurnal range of temperature, although this is more marked in the drier winter months, where minimum temperatures average 22 °C (71.6 °F).

Rainfall in the city averages around 2,500 millimetres (98 in) a year.