Weather will likely play an important role when you plan your trip to San Marcos, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually March and April with an average temp of 61°F, while the coldest months are January and December with an average of 60°F. The rainiest months in San Marcos are June, September, May, and August, with each month seeing an average of 19 inches of rainfall.