The Treatment Support Measure (TSM) Parent and Youth were created to help clinicians with actionable feedback when youths are not making sufficient progress in treatment. This study examined the psychometric properties of the Dutch TSM Parent and TSM Youth. Parents (n = 172) and youth (n = 122) were recruited at 2 outpatient mental health care institutions. Children of participating parents (50.6% boys) had a mean age of 11.9 years (SD = 3.46; range 4-18). Participating youth (30.3% boys) had a mean age of 15.68 years (SD = 1.75; range 12-18). Participants were asked to complete the TSM and questionnaires measuring related constructs once during treatment. Responses to the TSM Parent items were explained by 9 instead of 5 subscales, and responses to the TSM Youth items were explained by 8 instead of 4 subscales. The internal consistency reliability of both the TSM Parent and the TSM Youth scales was generally good. The convergent validity of the TSM Parent and the TSM Youth was also good, although the divergent validity was less convincing. The criterion validity was inconclusive; the TSM Parent was not able to differentiate between problematic and nonproblematic treatments, but multiple scales of the TSM Youth were able to differentiate between these groups. The TSM Parent and TSM Youth have potential to be helpful tools in clinical practice. They could signal potential barriers to youth progress and direct the conversation between the clinician and youth and parents about adaptation of treatment. KEY PRACTITIONER MESSAGE: This is the first study to investigate the psychometric properties of the Treatment Support Measure (TSM) Parent and Youth versions, which are created to help clinicians with actionable feedback when youths are not making sufficient progress in treatment. The Dutch TSM Parent and TSM Youth have moderate to good psychometric properties. The Dutch TSM Parent and TSM Youth might be helpful tools for use in clinical practice: they contain variables that are related to youth outcome, can signal ...