The Adult Prosocialness Behavior Scale (APBS) is most often used to measure adult prosociality. We conducted a reliability generalization meta-analysis to compute the average APBS reliability and examine the heterogeneity among reliability estimations and the influence of moderator variables. An exhaustive search identified 74 articles that applied the APBS with 16 items assessed on a 5-point Likert-type scale. Of these, 58 had reliability coefficients with the current data, and 76 reliability estimates were provided. Random- and mixed-effects models were used. The average reliability coefficient was .903 for Cronbach's alpha, .896 for McDonald's omega, and .674 for test-retest. Moderator analyses were used to create a predictive model in which the target population and study language accounted for 48.7% of the total variability among Cronbach's alpha coefficients. Although the APBS has shown satisfactory internal consistency, it can vary as a function of several factors.