Objective: The purpose of this study was to begin developing and validating an instrument to measure self-trust in adults. Method: Using a multi-phased and mixed methods approach to instrument development, the RSTI scores were tested across three samples reflecting national demographics of adults in the United States (n = 157, n = 692, n = 261). The 3 studies included construct conceptualization, development and pilot testing of initial items, exploration of factor structure through EFA and parallel analysis, confirmation of factor structure using CFA, and assessment of reliability and validity evidence. Results: The RSTI was composed of 26 items extracted into five factors, which demonstrated largely adequate model fit, good internal consistency, and adequate validity. Conclusion: This research provided an operational definition of self-trust, a new measure self-trust in adults, and a framework for counselors to investigate the role of self-trust in client conceptualization and treatment.