Thailand has three distinct seasons: dry, hot and wet. Thailand has a large geographical spread from North to South and has many variations from the mountains around Chiang Mai to the tropical islands of the Andaman Sea and the Gulf of Thailand. Koh Phuket weather is therefore different than other areas of Thailand.
Typically mid December through March when the weather is usually sunny and 30C/85F.
This season starts in April and lasts until mid to late September. The tropical sun is typically brighter and hotter during this period.
Phuket is a tropical island and you can expect some rain at anytime in any month. However, the wet season is October through to mid December, though it is still warm and there are generally many sunny days.