BackgroundThere is a strong evidence base for the benefits of parenting interventions for parents without severe mental illness (SMI). As the impact of maternal SMI can be significant on child development, mothers need support to maximise outcomes for themselves and their children. Some mothers with SMI require admission jointly with their baby to a Mother and Baby Unit (MBU), a psychiatric inpatient ward, for assessment and treatment. However, MBUs do not yet offer formally evaluated, evidence-based parenting interventions as a matter of routine. This paper describes a study to investigate the feasibility and acceptability of conducting a randomised controlled trial (RCT) to evaluate a parenting and psychological intervention targeting the mother’s and infant’s wellbeing for mothers admitted to a MBU.Methods/DesignThis study is a multisite, single-blind feasibility trial with half the participants randomised to the Baby Triple P Positive Parenting Programme plus treatment as usual (TAU) and the other half randomised to TAU alone. Self-report and observer-rated assessments are collected at baseline, 10 weeks post-baseline and 6 months post-baseline. Participants are mothers admitted to a MBU in the Northwest of England or the Midlands. Participants are included if they are fluent in English to provide informed, written consent. Our objective is to determine whether we can recruit 66 women, randomise 60, and retain them in the intervention and study, and whether the intervention and study procedures are acceptable. As part of a nested process evaluation, qualitative interview data from trial participants and MBU staff will inform feasibility and acceptability. The feasibility of collecting data required to conduct an economic evaluation of the intervention will also be explored.DiscussionAlthough research has been conducted in relation to mothers with severe mental illness and MBUs, to our knowledge, this is the first controlled trial to test the feasibility, acceptability, uptake and retention alongside the potential efficacy of a parenting intervention for this population. This study is essential to examine the contextual challenges involved in this setting with this population and to identify any refinements required.Trial registrationISRCTN12765736. Date of first registration: 2 February 2017.Electronic supplementary materialThe online version of this article (10.1186/s13063-018-2869-z) contains supplementary material, which is available to authorized users.