May to October is wet — but Bangkok in the rains has its own appeal, lower prices, and is very manageable.
Rain typically falls in heavy afternoon showers (1–3 hours) rather than all-day drizzle; mornings are usually clear; this matters a lot for planning.
Do outdoor sightseeing 8–11 AM; have lunch; rest during the rain (1–4 PM); go out again 4–7 PM; evenings are usually fine.
A small fold-flat umbrella (not a poncho) is most practical; waterproof sandals or shoes; quick-dry clothes; waterproof phone case for very heavy showers.
30–40% lower hotel rates, fewer tourists at temples, greener and more lush cityscape, cooler than the hot season; the rains don't ruin Bangkok.
Some lower-lying streets (Sukhumvit sois, Silom backstreets) flood briefly after very heavy rain — easily navigated with waterproof sandals; not dangerous.
Choosing accommodation and tours with flexible cancellation is worth it during rainy season, since a heavy downpour can occasionally reshuffle same-day plans.
A compact umbrella, quick-dry clothing, and a dry bag for electronics cover most rainy-season needs — a full rain jacket is rarely necessary given the heat and humidity.