Cambodia's climate, like that of the rest of South and Southeast Asia, is dominated by the southwest monsoon. Accordingly, it has two seaons, dry season and rainy season, the latter from mid-May to mid-October, which is the tourism's off-season. During the wet season rain showers are usually heavy, but not lasting. There are sunshine hours during the rainy season, too.

During the dry season months from November to April, there is much less rainfall, but it can occur. The river and lake levels sink and the soils get drier from month to month. In April and begin of May the air can be quiet dusty. From November to March the winds from the northeast are dry and cooler, they are called anti-monsoon. The southern part of the country, where the beach resorts are located, has a shorter dry season than the inland areas.

Temperatures are fairly uniform in the central plains of Cambodia, with an annual average around 25 °C (77.0 °F). January is the coolest and April is the hottest month. Maximum temperatures of higher than 32 °C (89.6 °F), are common, in April and begin of May they may rise to more than 38 °C (100.4 °F). Minimum temperatures rarely fall below 10 °C (50 °F).

The total annual precipitation average is between 1,000 and 1,500 millimeters (39.4 and 59.1 inch). The heaviest amounts fall along the southwest coast in front of Cambodia's highest mountain range, the Cardamom mountains, with 2,500 up to 5,000 millimeters.

Similar to other tropical countries, the relative humidity is high throughout the year, usually exceeding 90 percent at nighttime and about 50 percent during the daytime.

Some degree of flooding occurs almost every year. Tropical cyclones that often strike the coastline of Vietnam between August and October can cause heavy rainfall and floodings in Cambodia, too, but the storms are less devastating here.