Sepsis is an increasingly common presentation to which ambulance resources are dispatched. Whilst early administration of antibiotics (AB) has been associated with improved outcomes, the impact of prehospital administration in an ambulance service context appears uncertain. This systematic review aimed to compare the effect of prehospital administration of antibiotics together with usual care (oxygen and intravenous fluids), to usual care alone, on mortality for patients with sepsis. A systematic review was conducted adherent to JBI methodology. Studies were eligible for inclusion if they were published after 2000; conducted in the prehospital setting; compared AB plus usual care to usual care alone in the prehospital phase; and reported an outcome of mortality at any time point. Systematic searches of Medline, CINAHL, EMBASE and Google Scholar were conducted, with included articles subjected to quality assessment using JBI appraisal tools. Each stage was completed by two authors, with a third engaged to resolve conflicts. A narrative synthesis was conducted and reported, and certainty of evidence was assessed. Of 587 studies identified from the searches, five satisfied the inclusion criteria and were included in the data synthesis. Two were randomized controlled trials, and three used observational comparative designs assessed as being at low-to-moderate risk of bias. Regarding the primary outcome of mortality, there was no evidence from high-quality studies with a low risk of bias that prehospital administration of ABs decreased mortality when measured at 28, 30 or 90 days. Regarding secondary outcomes, there was no evidence from high-quality studies with a low risk of bias that prehospital ABs reduce the length of stay in the hospital generally or the intensive care unit. The certainty of findings was low for mortality at 90 days, and very low for measurement at 28 and 30 days. There was insufficient evidence from high-quality studies with a low risk of bias indicating prehospital administration of ABs in addition to usual care, compared to usual care alone, reduces mortality at 28, 30 or 90 days, or length of stay in hospital or ICU, for adult patients with sepsis. There is insufficient evidence to enable the recommendation of routine administration of antibiotics to patients with sepsis presenting to ambulance service clinicians in the prehospital setting. Investigation of administration to more severe sepsis presentations in settings where prolonged prehospital intervals are inherent is warranted.