BackgroundEmotional recognition is a key aspect of mentalizing other people’s minds. People with Williams syndrome (WS) are reported to be hypersocial and empathetic; however, few studies have investigated their emotion processing ability. AimsThis study aimed to examine emotion knowledge in people with WS and to further understand their emotion-related mentalizing ability. Methods and ProceduresSix basic emotions (happiness, sadness, anger, fear, disgust, and surprise) were tested by narrating scenarios targeting specific emotions. Participants with WS (n = 16, 8F/8 M, CA = 8.46, MA = 5.27) listened to each scenario and pointed to an emoticon displayed on a card. Correct percentages of each target emotion and replacement by other emotions were analyzed. Comparison groups comprised mental age-matched controls (n = 16, 8F/8 M, CA = 5.26) and chronological age-matched controls (n = 16, 8 F/8 M, CA = 8.31), sixth graders (n = 20, 10F/10M, CA = 12.04), and college students (n = 20, 10F/10M, CA = 20.03). All were Chinese-speaking participants with right handedness. Outcomes and ResultsPeople with WS were delayed compared to the mental age-matched group but differed from the chronological age-matched group in emotion processing. The 6th graders showed different patterns from college students, suggesting that development of emotion processing takes time. Conclusions and ImplicationsThese findings suggest (1) a gap in the maturation of emotion knowledge between people with WS and typically developing controls, (2) a nonequivalent chronological emotion-related mentalizing ability in people with WS, and (3) developmental changes in emotion processing from childhood to adulthood.