Sanya is genuinely tropical: warm and dry in winter, hot and wet in summer, with the wettest months August and September overlapping with the typhoon season. Daytime temperatures rarely drop below 20°C even in January and routinely hit 33-34°C from June to September.
November to April is the peak season and the best window for a beach holiday — clear skies, low humidity, warm water, and only occasional rain. This is also when the city is busiest with domestic Chinese tourists escaping the cold of the mainland. The downside is the prices: Chinese New Year (late January or February), the National Day Golden Week (first week of October), and the Lunar New Year period are at-all-cost peaks. Book months ahead or avoid those weeks.
May and October are shoulder months, generally pleasant with occasional rain, fewer crowds, and substantially lower prices.
June to September is the wet season. Days can be hot and humid with thunderstorms in the afternoon. Typhoons are a real risk — they pass through every couple of weeks at the peak of the season and occasionally hit Sanya directly. Hotels become much cheaper but flights can be disrupted.
The water stays warm enough to swim year-round (24-29°C). For diving and snorkelling, visibility is best from April to June.