Background: The expression of MDM4, a well-known p53-inhibitor, is positively associated with chemotherapy response and overall survival in epithelial ovarian cancer (EOC). The basis of this association remains elusive. Since the occurrence of metastasis is one of the factors responsible for the high death rate of this cancer, we analyzed MDM4 involvement in EOC metastatic process. Methods: In vivo and in vitro models, based on 2D and 3D assays, were employed to assess the activity of MDM4 in ovarian cancer progression. A 3D-bioprinting co-culture system was ad hoc developed for this study. Proteomic analysis was conducted on 3D multicellular tumour spheroids to assess pathways triggered by MDM4 overexpression. Results: In mouse models, increased MDM4 reduced intraperitoneal dissemination of human and murine EOC cells, independently of p53 and in a cell-autonomous way. Consistently, high MDM4 correlates with increased overall survival probability in large public data sets. 2D and 3D assays indicated that MDM4 impairs the early steps of the metastatic process. The 3D-bioprinting co-culture system showed reduced dissemination and intravasation into vessel-like structures of MDM4-expressing cells. Proteomic analysis of EOC spheroids revealed that MDM4 reduces protein synthesis and decreases mTOR signaling. Accordingly, MDM4 did not further inhibit EOC cell migration when its activity towards mTOR is blocked genetically or pharmacologically. Conversely, increased MDM4 reduced the efficacy of mTOR inhibitors in constraining EOC cell migration. Conclusions: Overall, these data clarify the antagonism of MDM4 towards EOC progression and suggest the usefulness of MDM4 assessment for tailored application of mTOR targeted therapy.