Summary This study examined the trends in reference crop evapotranspiration (ET0) on monthly and annual time scales in Iran. ET0 was estimated using the globally accepted Food and Agriculture Organization (FAO) Penman Monteith method (FAO-56 PM) over the 16 weather stations located in the different regions of Iran. The trends in ET0 were detected by using the Mann–Kendall (MK) test after the removal of the significant lag-1 serial correlation effect from all the ET0 time series by pre-whitening. The slopes of trend lines were computed using the Theil–Sen’s slope estimator. The spatial and temporal homogeneity of trends were tested as well. The multiple regression analysis was performed in each time series of the governing meteorological variables to identify the cause of observed trends in ET0. Results showed that both statistically significant increasing and decreasing trends were observed in the annual and monthly ET0. The increasing trends in ET0 were more pronounced than the decreasing trends. In annual time scale, the strong positive (negative) trend in ET0 over Iran of the magnitude of about 186 (−65) mm/year per decade was observed. In monthly time scale there was greater number of increasing trends than that of the decreasing trends in most of the warm months. The most strong positive (negative) trend magnitude was found in April (July) with Theil–Sen’s slope equal to 14 (−8.7) mm/year per decade. The results of homogeneity test indicated no homogeneity in ET0 trends between the stations and months when the entire study domain is considered. Wind speed was found to be the most dominant variable influencing ET0 in all the months except the winter months in Iran.