Phuket's weather is shaped by two seasonal monsoon systems, and understanding the basic mechanism explains most of what visitors actually notice through the year.
The south-west monsoon (roughly May–October)
Brings the bulk of the island's rain and rougher seas, particularly affecting the west-facing coast where most tourist beaches sit. Rain tends to arrive in shorter, heavier bursts rather than constant all-day drizzle, leaving plenty of dry hours on most days even during the wettest months.
The north-east monsoon (roughly November–February)
Brings Phuket's driest, most reliably sunny stretch — this is the core of high season for exactly that reason, and also when seas are calmest for island-hopping and diving.
Shoulder months
March-April and October-November sit between the two systems and can go either way — often good conditions with lower crowds than peak season, but less predictable than either core season.
Why this matters for planning
See our best time to visit guide for how this translates into practical trip planning, and our Similan Islands guide for how the monsoon directly affects specific tour availability.