Objective: This in vitro study evaluated the effect of incorporating stannous chloride (SnCl₂) and sodium trimetaphosphate (TMP) into resin modified glass ionomer cement (RMGIC) on its mechanical properties, fluoride release, antimicrobial/antibiofilm activity, and cytotoxicity. Methods: Six experimental cements were tested: (1) RMGIC; (2) RMGIC+14 %TMP; (3) RMGIC+400 ppm SnCl₂; (4) RMGIC+ 14 %TMP+400 ppm SnCl₂; (5) RMGIC+800 ppm SnCl₂; (6) RMGIC+14 %TMP+800 ppm SnCl₂. Degree of conversion (%DC), diametral/compressive/flexural tensile strength (DTS, CS and FS), Vickers hardness surface (VHS) and surface roughness (Ra) were determined. Fluoride release (F) was assessed over 15 days in demineralizing/remineralizing solutions. Antimicrobial/antibiofilm effects were analyzed using agar diffusion test and DAPI Staining against Streptococcus mutans (S. mutans). Cytotoxicity was evaluated on human primary gingival fibroblasts/epithelial cells. Results: The incorporation of TMP and SnCl₂ enhanced the mechanical performance of the RMGIC. The group containing TMP with 400 ppm SnCl₂ showed the highest DTS and CS (p < 0.05). Ra and VHS varied slightly among groups but remained within acceptable ranges. F release peaked within the first 24 h, particularly in formulations containing TMP with SnCl₂ (p < 0.05), and declined gradually thereafter. SnCl₂, alone or combined with TMP, significantly reduced S. mutans viability (p < 0.05), as confirmed by bacterial counts and fluorescence staining. Cytotoxicity remained low in all groups containing SnCl₂. Significance: The modification of RMGIC with SnCl₂ and/or TMP showed potential to enhance certain functional properties. Biocompatibility concerns with TMP and the short evaluation period limit definitive conclusions. Further long-term studies are needed to confirm safety and clinical efficacy, particularly regarding fluoride release and antimicrobial effects.