Background: More than 90% of advanced ovarian cancer patients develop malignant ascites, which describes a buildup of fluid in the peritoneal cavity caused by increased vascular permeability and obstructed lymphatic drainage. Malignant ascites contains spheroidal three-dimensional tumor cell clusters which can contain stromal cells, cancer-associated fibroblasts, and blood cells. These ascites-derived spheroids facilitate peritoneal metastasis and treatment resistance; however, little is known about the phenotypic and proteomic changes caused by the ascites environment on these spheroids. Furthermore, the suitability of standard cell culture medium for ex vivo treatment response testing in personalized medicine approaches remains unknown. (2) Methods: Using mass spectrometry, we compared the proteome profiles of cell-free ascites to serum from ovarian cancer patients, followed by the proteome profiles of immortalized cancer cells cultured in malignant ascites versus standard cell culture medium as monolayers and spheroids. The effects of this fluid on the phenotype, molecular composition, and ex vivo chemotherapy responses of cancer cells were also measured. (3) Results: The proteome analysis of cell-free ascites identified increased abundances of extracellular, secreted, and membrane proteins compared to serum. Ascites promoted cell viability and spheroid formation of immortalized ovarian cancer cell lines when compared to cell culture medium. Despite altered baseline viability, growth of spheroids in ascites versus cell culture medium did not hinder chemotherapy response assessments, which could simplify future personalized medicine approaches. The observed phenotypic changes of cells grown in ascites could not be recapitulated by the addition of chemokines or periostin to cell culture medium alone, indicating that additional factors are necessary to induce the native phenotype. However, elevated levels of transglutaminase 2 (TGM2)—an ECM modulatory protein that enabled treatment resistance and disease spread— was identified in SKOV-3 2D and 3D cultures grown in ascites, indicating a direct role of ascites in the regulation of protein expression in cancer cells.