Background: Mesenchymal stem cells (MSCs) have shown positive therapeutic effects on various diseases; however, their functionality can decline during in vitro expansion. Selenium (Se) supplementation has emerged as a strategy for enhancing MSC culture. This study evaluated the effects of different forms of selenium (Na₂SeO₄, SeMet, ebselen, and chitosan-coated selenium nanoparticles (CS-SeNPs)) on the biological functions of MSCs. Methods: Human umbilical cord-derived MSCs (HUC-MSCs) were cultured in media supplemented with various selenium compounds at specific concentrations. To investigate their biological effects, we assessed cell proliferation, morphology, surface marker expression, and differentiation potential. Furthermore, to elucidate the underlying mechanisms, we analyzed key markers of cellular senescence, including p16, p21, IL-6, IL-8, p27, p53, and reactive oxygen species (ROS) levels. Results: All the selenium treatments promoted hUC-MSC proliferation at specific concentrations. CS-SeNPs and Na₂SeO₄ exhibited relatively high bioavailability, whereas ebselen and SeMet demonstrated relatively low toxicity. The optimal concentration (0.5 μM CS-SeNPs or 0.25 μM Na₂SeO₄) significantly enhanced proliferation without altering the hUC-MSC morphology, phenotype, or differentiation capacity. Both CS-SeNPs and Na₂SeO₄ effectively promoted hUC-MSC proliferation and reduced the senescence of hUC-MSCs by downregulating key senescence-related effectors: the cell cycle inhibitors p16, p21, p27, and p53; and the levels of ROS and senescence-associated secretory phenotype factors (IL-6 and IL-8). Conclusions: Selenium supplementation is an effective strategy for improving MSC expansion and alleviating senescence. The beneficial effects are dependent on the specific selenium compound used, with CS-SeNPs and Na₂SeO₄ showing particularly strong potential for enhancing the bioavailability and function of hUC-MSCs during in vitro cultivation.