Radically open-dialectical behavior therapy (RO-DBT) has been developed as an intervention to treat difficulties associated with over-control (OC). This study examined the effectiveness of a new RO-DBT-informed group intervention for OC. Participants were adults (n = 117) attending a psychiatric hospital for mental health difficulties. They were recruited to radical openness (RO; n = 58) or, if RO was full, they were placed on a waiting list and received Treatment as Usual (TAU, n = 59). RO participants completed measures pre and posttreatment and TAU participants completed the measures at the same time points. RO participants continued to receive TAU and completed measures at 3-month follow-up. The primary outcome measure was overall psychopathology, assessed using the Global Severity Index on the Brief Symptom Inventory (BSI). Secondary outcome measures related to OC as well as skills use. Compared with TAU, the RO participants experienced statistically greater improvements in overall psychopathology. Statistically greater improvements were also found on secondary outcomes: (a) the Social Safeness and Pleasure Scale, (b) Dialectical Behavior Therapy (DBT) Ways of Coping Checklist, and (c) the Desire for Structure subscale on the Personal Need for Structure Scale. At 3-month follow-up, gains were maintained for skills use. Findings from this study provide preliminary support for the effectiveness of this treatment innovation in reducing psychopathology in patients with OC.