Frequency analysis of rainfall is a fundamental tool for the planning, design and operation of water resource, irrigation and drainage systems, particularly in arid and hyper-arid regions where rainfall is scarce, highly variable and frequently equal to zero in several months of the year. This study evaluates the one-parameter exponential distribution as a model for the frequency analysis of monthly rainfall in Zabol, a hyper-arid station in south-eastern Iran, using 52 years (1969-2020) of monthly rainfall records from the Zabol synoptic station. The single parameter of the exponential distribution, α, was estimated by the method of moments as the reciprocal of the mean monthly rainfall, and the probability density, cumulative distribution and quantile functions were derived for each month. Goodness of fit was assessed using the mean relative deviation (MRD) and the mean square relative deviation (MSRD) between observed and computed values, and the resulting quantiles were compared with those obtained from the two-parameter gamma and log-Pearson type III distributions, which are more commonly used in hydrological frequency analysis. The exponential distribution was found to describe the monthly rainfall of Zabol reasonably well, particularly once months with a high proportion of near-zero observations were treated separately, and its quantile estimates were broadly comparable with those of the two-parameter gamma and log-Pearson type III distributions while requiring the estimation of only a single parameter. Given its mathematical simplicity, the exponential distribution is proposed as a practical, low-data alternative for the frequency analysis of monthly rainfall in hyper-arid environments, complementing rather than replacing more heavily parameterised distributions.