Objective: While ample research links health beliefs to physical health in both healthy and clinical populations, the specific health beliefs that drive this effect remain underexplored. Addressing this gap is critical for mechanistic examinations and targeted intervention development. Building on previous work, this study aimed to assess subjective beliefs about immune system efficacy and develop a novel Perceived Immunity scale., Methods: Across three studies (total N = 378; Study 1: N = 206 healthy; Study 2: N = 132 healthy and N = 40 with immune-related diseases; and Study 3: subset of N = 87 from Study 2), we examine Perceived Immunity scale's validity, reliability, and association with everyday physical health., Results: Perceived immunity demonstrated excellent internal consistency (Cronbach alpha = 0.9), strong construct validity (all items were loaded onto one factor) and structural validity, including convergence validity with scales assessing subjective health perceptions, and discriminant validity from scales measuring external health control. These findings were replicated across different cohorts. Furthermore, regression analyses revealed significant correlations with daily physical health measures, specifically somatic symptoms, across cohorts (all β > -0.347, all p < .001) and sick leave days (all β = -0.174, all p < .045). These associations were stronger in individuals with immune-related conditions. Finally, Perceived Immunity exhibited good test-retest reliability (interclass correlation coefficient = 0.74) and prospectively predicted somatic symptoms over time (β = -0.324, p = .002), with significant results observed up to 1 year., Conclusions: The Perceived Immunity scale offers a valuable tool for researchers and health care providers, providing insights into the interplay between specific health beliefs and daily health. Furthermore, its validation lays the groundwork for targeted interventions that explore how health perceptions may directly influence actual physical experiences., (© 2024 The Author(s). British Journal of Health Psychology published by John Wiley & Sons Ltd on behalf of British Psychological Society.)