Positive aspects of self-compassion (i.e., self-kindness and nonjudgmental acceptance of personal experiences) as well as negative aspects (i.e., high self-criticism and self-coldness) are strong predictors of anxiety, depression, worry, and quality of life. To date, however, relatively little is known about (a) how both aspects of self-compassion change during naturalistic treatment, (b) whether and how such changes relate to symptom improvement, and (c) which processes might explain the potential benefits of self-compassion. To address these gaps, the present study examined whether relations between changes in both aspects of self-compassion and treatment outcomes in a brief partial hospital setting for acute psychology could be explained by associated changes in repetitive negative thinking (RNT), an established maladaptive cognitive process involved in anxiety and depressive disorders. In a sample of 582 people receiving cognitive-behavioral (CBT) and dialectical behavior therapy over the course of 1–2 weeks, increases in positive aspects of self-compassion and decreases in negative aspects related to improvements in depression and anxiety. RNT mediated the relationship between decreases in negative aspects of self-compassion and improvements in anxiety and depression. However, a reverse model also showed that decreases in negative aspects of self-compassion could also explain relations between RNT and depressive symptom improvement only. These findings suggest that negative aspects of self-compassion and RNT may constitute important targets for treatment in acute settings. Future studies should investigate the impact of greater focus on self-compassion on RNT and symptom improvement using longitudinal experimental designs with multiple assessment points, examining causality and directionality.