Background <p>The Impostor Phenomenon Syndrome (IPS) is characterized by persistent self-doubt and fear of being exposed as intellectually fraudulent despite objective evidence of competence. Increasing evidence suggests that IP is common among health professions students, and demanding academic environments, together with early clinical exposure, have been associated with greater psychological distress. This study examined the severity of impostor phenomenon among undergraduate nursing and allied health students at Shahid Sadoughi University of Medical Sciences, Yazd, Iran.</p> Methods <p>This cross-sectional study was conducted in 2025 among 300 undergraduate students enrolled in Nursing, Midwifery, Operating Room Technology, Anesthesia Technology, and Medical Laboratory Sciences who had completed at least one clinical internship. These disciplines represent four-year undergraduate allied health degree programs. Participants were selected using stratified random sampling according to academic discipline. Impostor phenomenon was measured using the 20-item Clance Impostor Phenomenon Scale (CIPS), which assesses three dimensions: Fear of being exposed as a fraud (8 items), Discounting achievements (9 items), and Attributing success to luck (3 items). The total CIPS score ranges from 20 to 100 and is categorized as follows: 40 ≤ Mild IPS, 41–60 Moderate IPS, 61–80 Severe IPS, 80 &gt; Very severe IPS. Group comparisons were performed using independent-samples <i>t</i> tests, one-way ANOVA, or Mann–Whitney U tests, as appropriate. Associations between participant characteristics and impostor severity were explored using an ordinal logistic regression model. Statistical significance was set at <i>p</i> &lt; 0.05.</p> Results <p>Based on the conventional CIPS severity categories, 48.7% of students (<i>n</i> = 146) were classified as having severe IP symptoms, whereas 36.3% (109) experienced moderate symptoms. Only 7.0% of participants (<i>n</i> = 21) were classified as having mild symptoms, while 8.0% were categorized as having very severe symptoms (<i>n</i> = 24). The mean total CIPS score was 62.43 ± 14.06, placing the group means at the lower threshold of the severe symptom category. Among the CIPS dimensions, the highest mean score was observed for discounting achievements (28.26 ± 6.97), followed by fear of being exposed as a fraud (25.86 ± 5.69) and attributing success to luck (8.31 ± 2.81). Overall differences in impostor scores across disciplines were modest (p-value = 0.04), and analyses identified significant differences only for the attributing success to luck dimension (p-value = 0.01). Compared with Nursing students (reference category), Midwifery students (<i>p</i> = 0.004) and Anesthesia Technology students (<i>p</i> = 0.002) had significantly higher odds of belonging to higher impostor phenomenon severity categories. Operating Room Technology students showed a borderline association with higher severity (0.05), whereas no significant differences were observed for Medical Laboratory Sciences students compared with nursing (<i>p</i> &gt; 0.05).</p> Conclusion <p>Impostor phenomenon was highly prevalent among undergraduate nursing and allied health students, with the overall mean score falling within the severe range. Compared with Nursing students, those in Midwifery and Anesthesia Technology had significantly higher odds of experiencing more severe impostor phenomenon. However, the regression model explained only a small proportion of the variability, indicating that demographic and academic characteristics alone are insufficient to explain impostor experiences. These findings provide a foundation for future multicenter longitudinal and intervention studies to identify modifiable educational factors associated with impostor phenomenon and to evaluate strategies for reducing impostor-related distress during clinical training.</p> Trial Registration Number (TRN) <p>None.</p>