Background: Hospital-treated deliberate self-harm (DSH) is common, costly and has high repetition rates. Since brief contact interventions (BCIs) may reduce the risk of DSH repetition, we aim to evaluate whether a SMS (Short Message Service) text message Intervention plus Treatment As Usual (TAU) compared to TAU alone will reduce hospital DSH re-presentation rates in Western Sydney public hospitals in Australia. Methods/design: Our study is a 24-month randomized controlled trial (RCT). Adult patients who present with DSH to hospital emergency, psychiatric, and mental health triage and assessment departments will be randomly assigned to an Intervention condition plus TAU receiving nine SMS text messages at 1, 2, 3, 4, 5, 6, 8, 10 and 12-months post-discharge. Each message will contain telephone numbers for two mental health crises support tele-services. Primary outcomes will be the difference in the number of DSH re-presentations, and the time to first re-presentation, within 12-months of discharge. Discussion: This study protocol describes the design and implementation of an RCT using SMS text messages, which aim to reduce hospital re-presentation rates for DSH. Positive study findings would support the translation of an SMS-aftercare protocol into mental health services at minimal expense. Trial registration and ethics approval: This trial has been registered with the Australian and New Zealand Clinical Trials Registry (Trial registration: ACTRN12617000607370. Registered 28 April 2017) and has been approved by two Local Health Districts (LHDs). Western Sydney LHD Human Research Ethics Committee approved the study for Westmead Hospital and Blacktown Hospital (Protocol: HREC/16/WMEAD/336). Nepean Blue Mountains LHD Research Governance Office approved the study for Nepean Hospital (SSA/16/Nepean/170).