Background: Infant mental health represents a significant public health issue. The transition to parenthood provides optimal opportunities for supporting parenting competence. Especially parental mentalization, i.e. the caregiver’s ability to notice and interpret the child’s behavior in terms of mental states, is important in infancy where the caregiver-infant communication is based solely on the infant’s behavioral cues. Methods: This study evaluates the efficacy of the intervention Understanding Your Baby (UYB) compared to Care As Usual (CAU) in 10 Danish municipalities. UYB aims at promoting parental competence in new parents by supporting them in noticing their infants’ behavioral cues and interpreting them in terms of mental states. Participants will be approximately 1,130 singletons and their parents. Inclusion criteria are first-time parents, minimum 18 years old, living in one of the 10 municipalities, and registered in the Danish Civil Registration Register (CPR). Around 230 health visitors deliver the UYB as part of their routine observation of infant social withdrawal in the Danish home visiting program. During an interaction between the health visitor and the infant, the health visitor articulates specific infant behaviors and helps the caregivers interpret these behaviors to mental states. The study is a controlled parallel group study with data obtained at four time points in two phases: First in the control group receiving the publicly available postnatal care (CAU), secondly in the intervention group after UYB implementation into the existing postnatal services. The primary outcome is maternal competence. Secondary measures include paternal competence, parental stress, parental mentalizing, and infant socioemotional development. Analysis will employ survey data and data from the health visitors’ register. Discussion: Results will provide evidence regarding the efficacy of UYB in promoting parenting competences. If proved effective, the study will represent a notable advance to initiating the UYB intervention as part of a better infant mental health strategy in Denmark. Conversely, if UYB is inferior to CAU, this is also important knowledge in regard to promoting parenting competence and infant mental health in a general population. Trial registrationhttps://ClinicalTrials.gov with ID no. NCT03991416. Registered at 19 June 2019—Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT03991416