Weather will likely play an important role when you plan your trip to San Juan, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually May and April with an average temp of 81°F, while the coldest months are February and January with an average of 77°F. The rainiest months in San Juan are December, October, September, and July, with each month seeing an average of 13 inches of rainfall.