BACKGROUND: Meta-analyses indicate that 17% of mothers experience postnatal depression (PND) in the year following childbirth, with suicide the leading cause of direct maternal death between 6 weeks and 12 months postpartum. The consequences and costs of PND are particularly high due to impacts on infants as well as parents. If detected, PND usually responds well to psychological treatment and/or medication but national reports indicate > 50% of cases are undetected. To improve detection, we developed a digital screening system (CareLoop PND) whereby mothers use an app to monitor their mood daily using a validated measure (Edinburgh Postnatal Depression Scale; EPDS), with real-time responses uploaded to a secure server. In this paper, we describe the protocol of a study to determine feasibility of delivering a full-scale RCT comparing digital screening with standard NHS practice. METHODS: In this single-blind randomised feasibility trial, participants (n = 80) will be recruited during late pregnancy from two NHS maternity services and randomised (1:1) to receive CareLoop PND alongside their usual NHS care, or usual care alone. Those in the experimental arm will use the CareLoop PND app daily from ≥ 36 weeks' pregnancy until 8 weeks postpartum. During this period, participants scoring above EPDS screening thresholds (via the app or standard care) will be assessed to confirm diagnosis. True positive PND cases identified by digital screening will be referred to services for support. A blinded researcher will conduct follow-up assessments using clinical and health economic measures at 8 weeks and 6 months postpartum. At 8 weeks postpartum, experimental arm participants will also provide qualitative and quantitative feedback exploring app usability, acceptability, and implementation. Feasibility of delivering a full-scale RCT will be evaluated using a priori criteria relating to app engagement, study retention and completion of candidate primary outcome measures. CONCLUSIONS: Digital screening could increase appropriate referral to perinatal mental health care. However, prior to roll out in NHS services, evidence of efficacy and cost-effectiveness is needed. The current study protocol will determine whether a full-scale RCT examining efficacy and cost-effectiveness is feasible and will inform the design of such a trial. TRIAL REGISTRATION: Prospective ISRCTN registration (03/07/23): ISRCTN10781027; https://www.isrctn.com/ISRCTN10781027 .