Weather will likely play an important role when you plan your trip to San Marcos, but bear 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 temperature of 16°C, while the coldest months are January and December, with an average of 16°C. The rainiest months in San Marcos are June, September, May and August, with each month seeing an average of 472 mm of rainfall.