Hot and wet are the two adjectives used to describe the climate in Bangkok. The temperature never falls below 20°C and can reach up to 35°C. The monsoon season runs from May to November with varying degrees of rain during those months. The wettest month is October and if you are visiting during this time remember a raincoat and umbrella!
The rains are followed by a dry period between November and May. Temperatures are lowest until mid February followed by the highest temperatures March to May.
The period between November and March is the most popular for holidays to Bangkok, although you'll find that tourists visit all year round. Avoid peak season if you're on a budget and looking for cheap flights to Bangkok.