Precipitation is a principal element of the hydrological cycle and its temporal variability is important from both the scientific and practical point of view. The annual and seasonal precipitation trends of 41 stations in Iran for the period 1966–2005 have been analyzed using the Mann–Kendall test, the Sen’s slope estimator and the linear regression. The effective sample size method was applied to eliminate the effect of serial correlation on the Mann–Kendall test. The results indicated a decreasing trend in annual precipitation at about 60% of the stations. The decreasing trends were significant at seven stations at the 95% and 99% confidence levels. The magnitude of the significant negative trends in annual precipitation varied from (−)1.999mm/year at Zanjan station to (−)4.261mm/year at Sanandaj station. The spatial distribution of the annual precipitation trends showed that the significant negative trends occurred mostly in the northwest of Iran. On the seasonal scale, the trends in the spring and winter precipitations time series were mostly negative. The highest numbers of stations with significant trends occurred in winter while no significant positive or negative trends were detected by the trend tests in autumn precipitation. The significant negative trends ranged between (−)0.283mm/year at Zahedan station and (−)0.807mm/year at Sanandaj station in winter season. In addition, the highest and lowest significant increases of precipitation values were obtained over Semnan and Mashhad in summer at the rates of (+)0.110mm/year and (+)0.036mm/year, respectively.