Aleatory variability is the natural randomness in a process and can affect probabilistic seismic hazard assessment (PSHA). In this study, considering a simple case of a square areal source zone, I employ Monte Carlo methods to estimate aleatory uncertainties due to random variations in temporal, spatial, and magnitude distribution of seismicity within the zone for PSHA. The results show that (1) uncertainty from aleatory variability in PSHA is significant for areas with low‐seismic activity, (2) the ratio of the 85th to 15th percentiles of peak ground acceleration (PGA) decreases as the occurrence rate increases, and (3) accounting for random variations in seismic parameters changes the estimated PGA by more than 10%. My analysis applies to the case in which there are fewer than 10 earthquakes over 50 yr, the site is located outside of the areal source, and . This situation should be considered in PSHA due to the cutoff effect of the magnitude lower limit. In addition, the sensitivity analysis shows that random variations in earthquake magnitude distribution are the largest contributor to aleatory uncertainty in most cases.