The ubiquity of devices connected to the internet raises concerns about the security and privacy of smart homes. The effectiveness of interventions to support secure user behaviors is limited by a lack of validated instruments to measure relevant psychological constructs, such as self-efficacy – the belief that one is able to perform certain behaviors. We developed and validated the Cybersecurity Self-Efficacy in Smart Homes (CySESH) scale, a 12-item unidimensional measure of domain-specific self-efficacy beliefs, across five studies (𝑁 = 1247). Three pilot studies generated and refined an item pool. We report evidence from one initial and one major, preregistered validation study for (1) excellent reliability (𝛼 = 0.90), (2) convergent validity with self-efficacy in information security (𝑟SEIS = 0.64, 𝑝 < .001), and (3) discriminant validity with outcome expectations (𝑟OE = 0.26, 𝑝 < .001), self-esteem (𝑟RSE = 0.17, 𝑝 < .001), and optimism (𝑟LOT−R = 0.18, 𝑝 < .001). We discuss CySESH’s potential to advance future HCI research on cybersecurity, practitioner user assessments, and implications for consumer protection policy.