Introduction Approximately 30% of somatic hospital inpatients experience psychosocial distress, contributing to increased (re-)hospitalisation rates, treatment resistance, morbidity, and direct and indirect costs. However, such distress often remains unrecognised and unaddressed. We established ‘SomPsyNet’, a ‘stepped and collaborative care model’ (SCCM) for somatic hospital inpatients, aiming at alleviating this issue through early identification of distress and provision of appropriate care, providing problem-focused pathways and strengthening collaborative care. We report the protocol of the ‘SomPsyNet’ study, aiming to evaluate implementation and impact of the SCCM on distressed patients’ health-related quality of life. Secondary objectives include assessing efficacy of the screening procedures, influence of SCCM on other health outcomes and associated costs.Methods and analysis Our stepped wedge cluster randomised trial conducted at three tertiary hospitals comprises three conditions: treatment as usual (TAU) without screening for distress (phase 0), TAU with screening but without consequences (phase I, main comparator) and TAU with screening and psychosomatic-psychiatric consultations for those distressed (phase II). The time-of-transition between phases I and II was randomised. Sample size target is N=2200–2500 participants, with 6 month follow-up for distressed (anticipated n=640–700) and a subsample of non-distressed (anticipated n=200) patients. Primary outcome is mental health-related quality of life (SF-36 ‘Mental Health Component Summary score’); secondary outcomes include psychosocial distress, anxiety, depressive and somatic symptoms, symptom burden and distress, resilience, social support and qualitative of life, assessed by internationally accepted instruments, with good psychometric properties. Further, health claims data will be used to assess SCCM’s impact on direct and indirect costs.Ethics and dissemination SomPsyNet adheres to the Helsinki Declaration and is approved by the ‘Ethikkommission Nordwest- und Zentralschweiz’ (2019–01724). Findings will be published in peer-reviewed journals and communicated to participants, healthcare professionals and the public.Trial registration number Swiss National Clinical Trials Portal; ClinicalTrials.gov (NCT04269005, updated 19.09.2023).