Self-criticism is a psychological process largely studied as a vulnerability factor for several psychological difficulties in the adult population and, to a lower extent, in adolescent samples. Thus, the availability of instruments to effectively capture this construct is important, especially for younger populations. This study examined the factor structure, measurement invariance to group samples, and psychometric properties of the short version of the Forms of Self-criticism and Self-reassuring Scale (FSCRS-SF) in Portuguese adolescents in non-clinical and clinical samples. Two non-clinical samples (N = 1224 and N = 140) and a clinical sample (N = 103) were used. Participants' ages ranged between 12 and 18 years old for both genders. Participants voluntarily completed a set of self-report questionnaires in the classroom. 418 adolescents completed the FSCRS 6-weeks after the first administration. Confirmatory factor analyses showed that the factor structure of the FSCRS-SF was similar to the one found in the adults' version, with three factors (inadequate self, hated self, and reassured self). The measurement invariance was established for both samples. The FSCRS-SF revealed adequate to good construct validity, reliability, and temporal stability. Gender differences were found for the three subscales. Similarly, adolescents from the clinical sample reported higher levels of inadequate self and hated self and lower levels of reassured self in comparison with the non-clinical sample, as expected. The FSCRS-SF was revealed to be a valid and reliable measure of self-criticism and self-reassurance for adolescents and seems to be a useful tool for research and clinical purposes.