BACKGROUND: Infant mental health is a significant public health issue as early adversity and exposure to early childhood stress are significant risk factors that may have detrimental long-term developmental consequences for the affected children. Negative outcomes are seen on a range of areas such as physical and mental health, educational and labor market success, social network and establishing of family. Secure attachment is associated with optimal outcomes in all developmental domains in childhood, and both insecure and disorganized attachment are associated with a range of later problems and psychopathologies. In disadvantaged populations insecure and disorganized attachment are common, which points to the need of identifying early risk and effective methods of addressing such problems. This protocol describes an experimental evaluation of an indicated group-based parental educational program, Circle of Security-Parenting (COS-P), currently being conducted in Denmark. METHODS/DESIGN: In a parallel randomized controlled trial of two intervention groups this study tests the efficacy of COS-P compared to Care as Usual (CAU) in enhancing maternal sensitivity and child attachment in a community sample in the City of Copenhagen, Denmark. During the project a general population of an estimated 17.600 families with an infant aged 2-12 months are screened for two known infant mental health risks, maternal postnatal depression and infant social withdrawal. Eligible families (N = 314), who agree to participate, will be randomly allocated with a ratio of 2:1 into the COS-P intervention arm and into CAU. Data will be obtained at inclusion (baseline) and at follow-up when the child is 12-16 months. The primary outcome is maternal sensitivity. Secondary outcomes include quality of infant attachment, language, cognitive and socioemotional development, family functioning, parental stress, parental mentalizing and maternal mental wellbeing. DISCUSSION: The potential implications of the experimental evaluation of an indicated brief group-based parenting educational program to enhance parental sensitivity and attachment are discussed. TRIAL REGISTRATION: ClinicalTrials.govID: NCT02497677 . Registered July 15 2015.