Nha Trang's climate is unusual for Vietnam: the rainy season here runs from late September through December, while most of the rest of the country is dry. January to August is the best time to visit — dry, warm, and with minimal swell. The peak months of July and August bring the most domestic tourists (Vietnamese families and beach-goers from the cities) and prices rise accordingly.
February and March are often considered the sweet spot — the tourist season is under way but not yet packed, the sea is calm, and the light is good. The water temperature sits around 25–27°C year-round.
October and November are the wettest months, with heavy rainfall that can make the beach unpleasant for days at a time. The diving conditions also deteriorate in these months due to reduced visibility. If you arrive during rain season and the sea is rough, consider heading inland to Da Lat (3–4 hours by bus) until conditions improve.