Background: Sepsis in patients with cancer is increasingly common and associated with high mortality. To date, no studies have examined the effectiveness of prehospital antibiotics in septic patients with cancer. This study aimed without and to evaluate the effect of prehospital antibiotics in septic patients with cancer. Methods: We conducted a post-hoc sub-analysis of the PHANTASi (PreHospital ANTibioitcs Against Sepsis) trial database: a randomised controlled trial which enrolled patients with suspected sepsis who were transported to the emergency department by ambulance. Patients in the intervention group were administered prehospital intravenous antibiotics while those in the control group received usual care. We compared patients who had cancer to those who did not. Primary outcome was 28-day mortality; among the secondary outcomes, we included in-hospital mortality and 90-day mortality. Results: 357(13.4%) of the 2658 included patients had cancer in the past five years, of which, 209 (58.5%) were included in the intervention and 148 (41.5%) usual care groups; 28-day mortality was significantly higher in patients who were diagnosed with cancer in the past five years than those without cancer in the past five years: 15.2% vs. 7.1%, respectively (p < 0.001). Prehospital antibiotics in the group of patients with cancer in the last five years yielded no significant effect on survival. There were however, significantly fewer 30-day readmissions (p = 0.031) in the intervention group of cancer patients (12.2% vs 5.7%). Conclusion: Prehospital antibiotics did not improve overall survival. However, there was a significant reduction in 30-day readmissions.