ARTICLE ABSTRACTInflammation mediated by activation of JAK/STAT signaling is a major cause of chemotherapy resistance in cancer. We studied the impact of selectively blocking the IL6 receptor (IL6R) as a strategy to inhibit IL6-induced STAT activation and to overcome chemoresistance in pancreatic ductal adenocarcinoma (PDAC). To do this, STAT activation was investigated in tumors arising spontaneously in LSL-KrasG12D/+;LSL-Trp53R172H/+;Pdx-1Cre (KPC) mice. Plasma from patients with PDAC was assessed for its ability to activate STAT3/SOCS3 in human monocytes using immunofluorescence microscopy and quantitative gene expression assays. KPC mice and syngeneic mice (wild type and IL6−/−) implanted with KPC-derived cell lines were treated with an IL6R-blocking antibody (anti-IL6R). The impact of treatment on tumor growth in KPC mice and mice with KPC-derived tumor implants was monitored using ultrasonography and calipers, respectively. Tumors were analyzed by IHC to detect changes in STAT activation, tumor viability, and proliferation. We found that STAT3 was the most activated STAT protein in PDAC tumors from KPC mice. Plasma from patients with advanced PDAC stimulated STAT3/SOCS3 activation in human monocytes. In mice, anti-IL6R antibodies targeted Ly6Chi monocytes, inhibited STAT3 activation in tumor cells, and decreased tumor cell proliferation in vivo. IL6R blockade in combination with chemotherapy induced tumor cell apoptosis, tumor regressions, and improved overall survival. Overall, we show that IL6 signaling drives STAT3 activation in tumor cells and mediates chemoresistance in PDAC. Thus, disrupting IL6 signaling using anti-IL6R antibodies holds promise for improving chemotherapy efficacy in PDAC. Mol Cancer Ther; 16(9); 1898–908. ©2017 AACR.