Phuket has two broad seasons shaped by the Andaman monsoon.
High season — November to April
Dry, sunny, and the calmest seas of the year — ideal for island-hopping and diving trips. This is also when the island is busiest and prices peak, especially around Christmas/New Year and Chinese New Year.
Green season — May to October
The south-west monsoon brings heavier rain and rougher seas, particularly on the west-facing beaches. Rain tends to arrive in short, heavy bursts rather than all-day drizzle, leaving plenty of dry hours either side. Some boat trips to the Similan Islands pause entirely (the islands' national park typically closes seasonally). In exchange, hotel prices drop and the island feels far less crowded.
Shoulder months
Late October/November and April are often the sweet spot — the crowds and prices of low season, without the heaviest rains.