Background: Delivery of pulsed electric field (PEF) energy is used to destabilize cells through multiple biochemical processes. We questioned whether a specific PEF treatment type could synergize with anti-PD1 in an orthotopic Triple Negative Breast Cancer (TNBC) EMT6 mouse model. Our previous studies have demonstrated that PEF treatment induces neo-antigen presentation and triggers an innate and adaptive immune response. Anti-PD1 therapy is ineffective at blocking growth of (TNBC) EMT6 tumors that are orthotopically implanted in mice. We evaluated whether a biphasic monopolar form of PEF could synergize with anti-PD1 when it was administered intraperitoneally or intra-tumorally, and whether there was a change in immune response post treatment. Methods: Forty-eight female Balb/c mice were implanted with 200,000 EMT6 cells into the mammary fat pads. Once tumor diameter reached 5 mm they were randomized into 6 groups of 8 animals: 1) Sham/IgG; 2) intra-tumoral (IT) anti-PD-1; 3) intra-peritoneal (IP) anti-PD-1; 4) PEF + anti-PD-1(IP); 5) PEF + anti-PD-1(IT) 6) 2X PEF on day 0 and day 7 + anti-PD-1(IT administered weekly). Sham-PEF mimicked the same procedure but without PEF. Anti-PD1 was administered weekly, IT or IP at 200 µg/mouse and PEF was delivered once on day of randomization. Tumors were measured three times per week, and animal survival was monitored until tumor volume necessitated euthanasia (≥2000mm³). Blood was collected on day 14 for flow cytometric analysis of systemic immune response which included quantifying CD3 positive T cells, B cells and NK cells. Results: Anti-PD1 treatment (IT or IP) did not reduce tumor growth, nor did it prolong survival compared to Sham/IgG treated animals. All anti-PD1 alone (IT or IP) as well as Sham treated animals had to be euthanized by day 35. In contrast, PEF combined with anti-PD1 given IT and IP significantly reduced tumor growth by 83% and 70%, respectively, on Day 14 compared to anti-PD-1 only treated animals (p = 0.03). Interestingly, EMT6 bearing mice treated with double PEF + IT administered anti-PD1 survived longer compared to mice treated with single PEF + IT administered anti-PD1. On day 65, 71% of double PEF + IT administered anti-PD1 were still alive compared to 12.5% of mice treated with PEF + IT administered anti-PD1 (p = 0.002). Flow cytometry analysis on day 14 indicated an increase in circulating adaptive (T- and B-cells) and innate (NK cells) immunocytes in the PEF + anti-PD1 treated animals compared to the control. Conclusions: Collectively, these results indicate that some types of PEF ablation synergize with weekly anti-PD1 in stimulating an increase of CD3+ T cells, B cells and NK cells, reducing tumor growth and prolonging survival of tumor bearing mice. These results suggest the synergistic potential of combining Immunotherapy with some forms of PEF, which warrants further study in patients.