(For more in depth information, please read: Koh Rong Weather)
In general, Cambodia stays warm throughout the year. Middle of May to early November is the rainy season, with short bursts of very heavy rain during the day.
As such, many people like to visit Koh Rong between the middle of November and the start of May. During this time, temperatures can rise to as high as 40°C (March to May, with April being the hottest month).
Those that visit during the rainy season get to enjoy half-priced accommodation and beautiful rainy scenes. Because it never gets too cold in Cambodia, swimming in the ocean is still a very pleasurable experience (read: 5 Reasons to Visit Koh Rong in Rainy Season). The weather throughout the year is as follows:
November-February: Warm and dry. 25°C daytime.
March-May: Hot and dry. 35°C daytime.
June-August: Hot and wet. 35°C daytime.
September-Early November: Warm and wet. 25°C daytime.