Relaxation sensitivity indexes the fear of relaxation-related events. The purpose of this study was to develop and provide initial validation of a self-report measure of relaxation sensitivity, the Relaxation Sensitivity Index (RSI). Three independent samples of undergraduate students (n=300 unselected, n=349 non-clinical, and n=197 clinical analogs with elevated anxiety/depression symptoms) completed self-report measures to examine the factor structure, reliability, and validity of the RSI. Results of exploratory and confirmatory factor analyses supported a three-factor structure (correlated Physical, Cognitive, and Social Concerns). The RSI demonstrated good internal consistency and construct validity as evidenced by expected correlations with measures of anxiety and depression symptoms. The RSI showed good predictive validity in terms of a history of fearful responding to relaxation. RSI scores were significantly higher in the symptomatic compared to non-clinical sample. Results suggest the RSI is a valid and reliable measure that may be useful in clinical and research settings.