Past research has typically shown negative effects of self-doubt on performance and psychological well-being. We suggest that these self-doubt effects largely may be due to an underlying assumption that ability is innate and fixed. The present research investigated the main hypothesis that incremental beliefs about ability might ameliorate negative effects of self-doubt. We examined our hypotheses using two lab tasks: verbal reasoning and anagram tasks. Participants’ self-doubt was measured and beliefs about ability were measured after participants read articles advocating either for incremental or entity theories of ability. American College Testing (ACT) scores were obtained to index actual ability level. Consistent with our hypothesis, for participants who believed ability was relatively fixed, higher self-doubt was associated with increased negative affect and lower task performance and engagement. In contrast, for participants who believed that ability was malleable, negative self-doubt effects were ameliorated; self-doubt was even associated with better task performance. These effects were further moderated by participants’ academic ability. These findings suggest that mind-sets about ability moderate self-doubt effects. Self-doubt may have negative effects only when it is interpreted as signaling that ability is immutably low.