Ovarian cancer (OV) ranks among the deadliest gynecological cancer, known for its high risk of relapse and metastasis, and a general resistance to conventional platinum-based chemotherapy. Selenoprotein I (SELENOI) is a crucial mediator implicated in human hereditary spastic paraplegia. However, its role in human tumors remains poorly elucidated. Here, we comprehensively analyzed SELENOI expression patterns, functions, and clinical implications across various malignancies through the integration of bulk transcriptomics, cancer databases, and in vitro and in vivo experiments. Pan-cancer analysis indicated upregulated SELENOI expression across various cancers, correlating with augmented malignancy, suppressed tumor immunity and poor prognosis. Knockdown of caused G0/G1-phase cell cycle arrest and diminished aggressive cancer phenotypes in OV cells. Moreover, SELENOI inhibition augments ferroptosis and reverses the cisplatin resistance in OV cells by modulating Akt phosphorylation. Conversely, overexpression of in OV cells enhanced therapeutic sensitivity to cisplatin by upregulating Akt phosphorylation. Importantly, in vivo studies demonstrated that SELENOI inhibition suppressed ovarian tumor growth and enhanced cisplatin's anticancer effects. These findings highlight the significant role of SELENOI in OV by modulating ferroptosis and chemotherapy resistance. Targeting SELENOI represents a promising therapeutic approach to promote the efficacy of platinum-based chemotherapy in OV, particularly in cases of resistance.