Playa del Carmen has a sub-tropical climate, which means it's almost always warm. Year round the average temperature is about 27C or 80F. Although Playa del Carmen is usually sunny and warm, there are exceptions. Regardless of when you come you should expect great beach days and water warm enough to swim in.
Here is a summary of seasonal variations:
February to May
Spring is a beautiful time of year, with little rain and flowers everywhere. The sea is calmer and gets steadily warmer as temperature rises. Nights can still be cool, but a sweater is rarely needed.
June to August
The summer in Playa del Carmen is hot. The water is so warm you can stay in for hours. Blue skies prevail. There is some rainfall with short heavy showers in the afternoon or evening. Nights do not get much cooler.
September and October
This is the season for tropical storms and the occasional hurricane. It doesn't rain every day for two months, but there could be periods of more persistent rain. It's still warm.
November to January
These are the coolest months in Playa, which means it's very comfortable. There might be some rain, but it's not very frequent. This time of year nights can be cool - temperatures can drop to as low as 12C or 54F!