Short answer
Warm water and thinning crowds after the first week. Storms are still possible, so build in a buffer day.
01Weather
Highs around 30 °C, with humidity slowly easing towards the end of the month. Rain is still frequent, and tropical storms remain a real risk until October. When the weather is settled, late September can be wonderful.
02Holidays
National Day on 2 September usually creates a long weekend that fills the island. After it, schools are back and weekdays become quiet, with prices dropping.
03On the water
With the sea at 28–29 °C, swimming and kayaking are excellent. Deep water soloing is in season, and the bay is noticeably less busy than in July.
?Questions people ask
Is September a good time for Cat Ba?
After the first weekend, yes, for warm water and smaller crowds. Keep plans flexible because of the storm risk.
Can I still swim in late September?
Yes, the sea is still around 28 °C.