The following monthly temperatures are averages. Clearly, it will be warmer during the day, and cooler at night. The weather is very pleasant from November until May. Sometimes in June, and definitely from July until September, it is quite hot and humid. Mazatlan has a very intense rainy season. It's not recommended to visit between July and September. January through March are the coolest months, with typical daytime temperatures in the high 70's to low 80's and evening temperatures in the low 60's.
Due to the high water content of the night and morning air in Mazatlan, you should always wear a jacket or sweater at night. There have been many cases of bronchitis that result from walking around in a t-shirt or tank top at night.