Public risk perceptions of mass disasters carry considerable influences, both psychologically and economically, despite their oft-times imprecise nature. Prior research has identified the presence of an optimistic bias that affects risk perception, but there is a dearth of literature examining how these perceptions differ among cultures—particularly with regard to mass disasters. The present study explores differences among Japanese, Argentinean, and North American mental health workers in their rates of the optimistic bias in risk perceptions as contrasted between natural disasters and terrorist events. The results indicate a significant difference among cultures in levels of perceived risk that do not correspond to actual exposure rates. Japanese groups had the highest risk perceptions for both types of hazards and North Americans and Argentineans had the lowest risk perceptions for terrorism. Additionally, participants across all cultures rated risk to self as lower than risk to others (optimistic bias) across all disaster types. These findings suggest that cultural factors may have a greater influence on risk perception than social exposure, and that the belief that one is more immune to disasters compared to others may be a cross-cultural phenomenon.