December in Ko Samui averages a daily high of 29.4°C with nights around 24°C. Expect roughly 16.1 wet days totalling 308.9mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Ko Samui is January–April.
Based on KO SAMUI, TH (14.7km), 1995–2024. 23 years of December data.