Gilan has a humid subtropical climate with the heaviest rainfall in Iran: reaching as high as 1,900 millimetres (75 in) in the southwestern coast and generally around 1,400 millimetres (55 in). Rasht, the capital of the province, is known internationally as the "City of Silver Rains" and in Iran as the "City of Rain".

Rainfall is heaviest between September and December because the onshore winds from the Siberian High are strongest, but it occurs throughout the year though least abundantly from April to July. Humidity is very high because of the marshy character of the coastal plains and can reach 90 percent in summer for wet bulb temperatures of over 26 °C (79 °F). The Alborz mountains provides further diversity to the land in addition to the Caspian coasts.

The coastline is cooler and attracts large numbers of domestic and international tourists. Large parts of the province are mountainous, green and forested. The coastal plain along the Caspian Sea is similar to that of Mazandaran and mainly used for rice paddies.