Despite advances in immune checkpoint inhibitors, chemotherapy remains the standard therapy for patients with pancreatic ductal adenocarcinoma (PDAC). As the combinations of chemotherapy, including the FOLFIRINOX [5-fluorouracil, F; irinotecan, I; and oxaliplatin, O (FIO)] regimen, and immune checkpoint inhibitors have failed to demonstrate clinical benefit in patients with metastatic PDAC tumors, there is increasing interest in identifying therapeutic approaches to potentiate ICI efficacy in patients with PDAC. In this study, we report that neoadjuvant FOLFIRINOX-treated human PDAC tumors exhibit increased MEK/ERK activation. We also show elevated MEK/ERK signaling in ex vivo PDAC slice cultures and cell lines treated with a combination of FIO. In addition, we find that the KPC-FIO cells, established from repeated treatment of mouse PDAC cell lines with six to eight cycles of FIO, display enhanced ERK phosphorylation and demonstrate increased sensitivity to MEK inhibition in vitro and in vivo. Significantly, the KPC-FIO cells develop tumors with a proinflammatory immune profile similar to human PDAC tumors after neoadjuvant FOLFIRINOX treatment. Furthermore, we found that the MEK inhibitor trametinib enables additional infiltration of highly functional CD8+ T cells into the KPC-FIO tumors and potentiates the efficacy of anti-PD-1 antibody in syngeneic mouse models. Our findings provide a rationale for combining trametinib and anti-PD-1 antibodies in patients with PDAC after neoadjuvant or short-term FOLFIRINOX treatment to achieve effective antitumor responses.