The study examines differential motivational factors underlying risk information seeking and sharing in the context of transboundary air pollution (“haze”) in Singapore. In particular, it addresses specific conditions under which information insufficiency influences information seeking and sharing. The analysis of an online survey with 1,021 Singaporeans showed that one’s perceived knowledge about haze was a consistent predictor of both information seeking and sharing intention. Information seeking was attitudinally driven, whereas information sharing was primarily driven by the perceived capacity of sharing information. Others’ information insufficiency promoted information sharing only when individuals considered themselves sufficiently efficacious at and had positive attitudes toward information sharing. Addressing perceived others’ knowledge, beyond one’s own, this study provides new theoretical insights into the factors that may promote risk information seeking and sharing.