The Circle of Security – Parenting (COSP™) is a psychoeducational intervention aiming at fostering secure child-parent attachment relationships. In a randomized controlled trial, we investigate the effect of COSP™ as an adjunct to care-as-usual compared to only care-as-usual for at-risk families. Mothers and their 2–12-month-old infants were randomized into COSP™ +care-as-usual (n = 197) for at-risk families in Copenhagen or only care-as-usual (n = 100). At-risk status was either mothers diagnosed with postpartum depression and/or infants showed social withdrawal. The primary outcome was maternal sensitivity which was coded with the Coding Interactive Behavior. Our secondary outcomes were maternal reflective functioning, assessed with the Parental Reflective Functioning Questionnaire – Infant Version, and child-mother attachment, assessed with the Strange Situation Procedure. Results showed no significant differences between the RCT groups on either the primary or secondary outcomes (all ps ≥ .146). We discuss these findings in relation to the applicability and targeted population who can benefit from COSP™, and whether alternative programs would be more effective for at-risk families with infants.