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Atmospheric humidity has an important role in the energy balance of the planet Earth, and it is effective in explaining climate change and consequently global warming, which has been a major topic of discussion among scientists in recent decades. Therefore, in this study, the maximum and minimum annual and seasonal trends of the relative humidity of 41 synoptic stations in the country during the statistical period of 1960–2019 were evaluated. Three statistical tests were used to analyze the data trend: the Mann–Kendall statistical test, Sen's slope estimator, and linear regression. In the study area, the maximum and minimum annual humidity trends were positive for 83.26% and negative for 17.73% of the stations, respectively. The highest increase in maximum and minimum humidity values, respectively, includes Chabahar in Southeast and Dezful in Southwest stations with a rate of + 2.55 and + 2.10 °C per decade. At the seasonal scale, the highest decrease in maximum and minimum humidity was identified in the summer at Tabas station in the central part of Iran. The highest number of stations with a decreasing or negative trend in the seasonal time series of maximum and minimum humidity occurred in winter with 37 stations. In contrast, most stations had a significant increasing or positive trend in winter and fall. Overall, the results showed that although the rate of decreasing trend of humidity for the study period was higher than the rate of an increasing trend of humidity, there was a similar decreasing trend in the studied variables.