INTRODUCTION: Globally, approximately half of the estimated 6.3 million under-5 deaths occur in the neonatal period (within the first 28 days of life). Kenya ranks among countries with the highest number of neonatal deaths, at 20 per 1000 live births. Improved identification and management of neonates with potentially life-threatening illness is critical to meet the WHO's target of ≤12 neonatal deaths per 1000 live births by 2035. We developed an interactive (two-way) short messaging service (SMS) communication intervention, Mobile Solutions for Neonatal Health (Mobile women's and children's health (WACh) NEO), focused on the perinatal period. Mobile WACh NEO sends automated tailored SMS messages to mothers during pregnancy and up to 6 weeks post partum. Messages employ the Information-Motivation-Behaviour Skills framework to promote (1) maternal implementation of essential newborn care (ENC, including early, exclusive breast feeding, cord care and thermal care), (2) maternal identification of neonatal danger signs and care-seeking, and (3) maternal social support and self-efficacy. Participants can also send SMS to the study nurse, enabling on-demand remote support. METHODS AND ANALYSIS: We describe a two-arm unblinded randomised controlled trial of the Mobile WACh NEO intervention. We will enrol 5000 pregnant women in the third trimester of pregnancy at 4 facilities in Kenya and randomise them 1:1 to receive interactive SMS or no SMS (control), and conduct follow-up visits at 2 and 6 weeks post partum. Neonatal mortality will be compared between arms as the primary outcome. Secondary outcomes include care-seeking, practice of ENC and psychosocial health. Exploratory analysis will investigate associations between maternal mental health, practice of ENC, care-seeking and SMS engagement. ETHICS AND DISSEMINATION: This study received ethical approval from the University of Washington (STUDY00006395), Women and Infants Hospital (1755292-1) and Kenyatta National Hospital/University of Nairobi (P310/04/2019). All participants will provide written informed consent. Findings will be published in peer-reviewed journals and international conferences. TRIAL REGISTRATION NUMBER: NCT04598165.