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The small GTPase Rac1 (Ras-related C3 botulinum toxin substrate 1) is commonly overexpressed in cancer and high Rac1 levels are associated with poor patient outcomes in ovarian cancer. Rac1 drives multiple hallmarks of cancer such as proliferation, quiescence, epithelial-to-mesenchymal transition (EMT), and transcriptional programs relevant to tumor development, progression, and chemoresistance. EMT promotes metastatic dissemination, chemoresistance, and cancer stem cell properties. We find that modest overexpression of Rac1 in ovarian tumor cells causes morphologic changes consistent with EMT. Elevated Rac1 (4- fold) expression decreases colony number and increases colony area when compared to vector control cells. In addition, cell invasion through an artificial basement membrane is increased by 10-fold in cells overexpressing Rac1. An increase in mesenchymal markers (e.g., N-cadherin) and decrease in epithelial markers (e.g., E-cadherin) was observed as a consequence of Rac1 expression. Rac1 overexpression was associated with selective upregulation of Slug/Snai2 (8-fold) and Zeb2 (100-fold) mRNA levels with little impact on Snai1, Twist, or Zeb1. Slug/Snai2 expression is reportedly induced by oxidative stress and treatment of six ovarian tumor cell lines with tert-butyl hydroperoxide (TBHP) to generate oxidative stress increased the expression Slug/Snai2, Snai1, and heme oxygenase-1, an indicator of oxidative stress response. This finding confirms that elevated oxidative stress regulates certain EMT transcription factors in various ovarian cancer cells. Rac1 is an essential factor for activation of NADPH oxidases 1, 2, and 3 and generation of reactive oxygen species (ROS). Basal level of ROS was elevated in cells overexpressing Rac1 compared to vector control cells as detected by the fluorescent probe Dihydroethidium (DHE). Similarly, elevated Rac1 increased expression of the oxidative stress response genes heme oxygenase-1 (17-fold) and superoxide dismutase-1 (12-fold), further supporting that Rac1 stimulated ROS signaling in these cells. We further tested the impact of Rac1 inhibition in vivo using R-ketorolac. Analysis of tumor tissue from mice showed decreased expression of heme-oxygenase-1 and Slug/Snai2 in mice treated with R-ketorolac compared to placebo controls. R-ketorolac also decreased invasive implantation and expansion of ovarian cancer xenografts. Taken together, these findings illustrate the functional impact of Rac1 overexpression in ovarian cancer cells and suggest that Rac1 may be a viable target for further drug development to attenuate EMT in ovarian tumors. Citation Format: Martha M. Grimes, Dayna R. Dominquez, Michaela L. Granados, Alejandra Rosales, Melanie Rivera, Angela Wandinger-Ness, Laurie G. Hudson. Rac1 overexpression promotes epithelial-to-mesenchymal transition in ovarian cancer cells [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr B59.