Intensive short-term dynamic psychotherapy (ISTDP) is theorized to reduce negative affect by challenging patients' defense mechanisms so that they can experience and work through attachment-trauma-related emotions. While ISTDP has been shown to decrease depressive symptoms in single treatment-resistant depression (TRD), it has not been established whether negative affect and emotional repression are reduced, as theorized. Next to depressive symptoms, this retrospectively registered (https://osf.io/v46gy) randomized controlled trial, therefore, examined the effects of ISTDP on emotional repression and negative affect in adults with TRD. Eighty-six adults with major depressive disorder, who had not responded to at least one trial of antidepressants were randomized to 20 sessions of ISTDP (N = 43) or a waitlist control condition (N = 43). Mixed-effect models on the intention-to-treat sample showed that compared to the control condition, ISTDP resulted in significantly lower posttreatment levels of depressive symptoms (d = -1.73), emotional repression (d = -1.91), and negative affect (d = -1.45). Similarly, ISTDP resulted in significantly lower levels of depressive symptoms (d = -2.67), emotional repression (d = -2.69), and negative affect (d = -1.85) at the 3-month follow-up. These results support the evidence base of ISTDP by showing that it can decrease depressive symptoms, emotional repression, and negative affect in TRD. Future studies should assess whether these effects are specific to ISTDP. (PsycInfo Database Record (c) 2023 APA, all rights reserved).