Background: Out-of-hours palliative care is a priority for patients, caregivers and policymakers. Approximately three quarters of the week occurs outside of typical working hours, and the need for support in care of serious and terminal illness during these times is commonplace. Evidence on relevant interventions is unclear. Aim: To review systematically the evidence on the effect of out-of-hours specialist or generalist palliative care for adults on patient and caregiver outcomes, and costs and cost-effectiveness. Methods: A systematic review of peer-reviewed and grey literature was conducted. We searched Embase, MEDLINE [Ovid], Cochrane Library, CINAHL, Allied and Complementary Medicine [Ovid], PsycINFO, Web of Science, Scopus, EconLit (Ovid), and grey literature published between 1 January 2000 and 12th November 2019. Studies that comparatively evaluated the effect of out-of-hours specialist or generalist palliative care for adults on patient and caregiver outcomes, and on costs and cost-effectiveness were eligible, irrespective of design. Only English-language studies were eligible. Two reviewers independently examined the returned studies at each stage (title and abstract review, full-text review, and quality assessment). Results: We identified one eligible peer-reviewed study, judged as insufficient quality. Other sources returned no eligible material. The systematic review therefore included no studies. Conclusions: The importance of integrated, 24-hour care for people in line with a palliative care approach is not reflected in the literature, which lacks evidence on the effects of interventions provided outside typical working hours. Registration: PROSPERO CRD42018111041.