Boredom is a ubiquitous emotion that has strong behavioral and mental health impacts. Research suggests that how people experience and regulate emotions is influenced by their beliefs about them. What lay beliefs about boredom do people have? The present research sought to answer this question using a mixed-methods approach. In Study 1, we conducted a series of individual and focus-group interviews (N = 29) to explore how people evaluate boredom. In Study 2, we developed and validated a 15-item self-report measure, the Boredom Beliefs Scale (BBS), in Hong Kong Chinese (N = 231) and American (N = 498) samples. In Study 3, we examined the scale’s convergent, discriminant, and incremental validity in a British sample (N = 296). We identified three lay boredom beliefs—the extent to which people recognize the functions of boredom (boredom functionality), affectively dislike this emotion (boredom dislike), and believe its experience to be normal (boredom normalcy). The three-factor BBS was demonstrated to be a reliable and valid scale that showed meaningful relationships with measures of emotion beliefs, boredom, and well-being. Our findings enrich the current literature by introducing a new construct, boredom belief, which has both theoretical and applied significance.