Tenascin-C (TN-C) silencing is a promising gene-based oncotherapy strategy because of its restricted tissue expression and its role in tumor survival, metastasis, angiogenesis and immune escape. In this study, we have used the powerful genome editing tool, CRISPR/Cas9 to study the effect of TN-C downregulation in human triple negative breast cancer cell line, MDA-MB-231, and then formulated a chitosan-based non-viral gene carrier vector to enable safe and efficient in vivo application of the CRISPR/Cas9 plasmids. A carrier system was prepared by grafting oxytocin to the cationic chitosan by using a polyethylene glycol heterobifunctional linker. The resultant nanoplex has been characterized by FT-IR, electron microscopy and dynamic light scattering, and it was shown that it carried CRISPR/Cas9 plasmids successfully into the cells. In vivo study, Oxy-PEG-CS-TN-C CRISPR/Cas9 nanoplexes were intratumorally administered twice a week to the xenograft breast cancer model in athymic mice. The most significant reduction in tumor diameter was detected in the Oxy-PEG-CS-TN-C CRISPR/Cas9 nanoplex group (42%). mRNA expression of TN-C was suppressed by approximately 48% in the nanoplex group. The expression of TN-C protein was markedly decreased. As a result, in our study a chitosan-oxytocin conjugate has been formulated for the first time, and a new therapeutic approach in triple negative breast cancer has been developed by editing TN-C using CRISPR/Cas9 carried by this non viral system. This study was funded by Inonu University, Department of Scientific Resarch Projects (TOA-2019-1543).