Assessment of psychological distress is important, because it may help to monitor treatment effects and predict treatment outcomes. We previously developed the 48‐item Symptom Questionnaire (SQ‐48) as a public domain self‐report psychological distress instrument and showed good internal consistency as well as good convergent and divergent validity among clinical and non‐clinical samples. The present study, conducted among psychiatric outpatients in a routine clinical setting, describes additional psychometric properties of the SQ‐48. The primary focus is on responsiveness to therapeutic change, which to date has been rarely examined within psychiatry or clinical psychology. Since a questionnaire should also be stable when no clinically important change occurs, we also examined test–retest reliability within a test–retest design before treatment (n = 43). A pre‐treatment/post‐treatment design was used for responsiveness to therapeutic change, comparing the SQ‐48 with two internationally widely used instruments: the Brief Symptom Inventory (n = 97) and the Outcome Questionnaire‐45 (n = 109). The results showed that the SQ‐48 has excellent test–retest reliability and good responsiveness to therapeutic change, without significant differences between the questionnaires in terms of responsiveness. In sum, the SQ‐48 is a psychometrically sound public domain self‐report instrument that can be used for routine outcome monitoring, as a benchmark tool or for research purposes. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message The SQ‐48 is developed as a public domain self‐report questionnaire, in line with growing efforts to develop clinical instruments that are free of charge. The SQ‐48 has excellent test–retest reliability and good responsiveness to therapeutic change or patient progress. There were no significant differences in terms of responsiveness between the SQ‐48 and BSI or OQ‐45. The SQ‐48 can be used as a routine evaluation outcome measure for quality assurance in clinical practice. Providing feedback on patient progress via outcome measures could contribute to the enhancement of treatment outcomes.