Women tend to be more accurate in decoding facial expressions than men. We hypothesized that women’s better performance in decoding facial expressions extends to distinguishing between authentic and nonauthentic smiles. We showed participants portrait photos of persons who smiled because either they saw a pleasant picture (authentic smile) or were instructed to smile by the experimenter (nonauthentic smile) and asked them to identify the smiles. Participants judged single photos of persons depicting either an authentic or a nonauthentic smile, and they judged adjacent photos of the same person depicting an authentic smile and a nonauthentic smile. Women outperformed men in identifying the smiles when judging the adjacent photos. We discuss implications for judging smile authenticity in real life and limitations for the observed sex difference.