<p>This study investigated whether the various beneficial properties of flowable composite resin could be maintained upon adding strontium-modified phosphate-based glass (Sr-PBG) as an antibacterial agent. The experimental composite resin groups contained 1, 3, 5, or 10&#xa0;wt% Sr-PBG. The mechanical, depth of cure, ion release, color analysis, surface gloss, antibacterial, and biocompatibility properties of the experimental groups were compared to those of the control group (100% resin). All experiments were performed before and after thermocycling to investigate the effects of thermal aging. Data were analyzed using one-way analysis of variance and Tukey’s tests (<i>p</i> &lt; 0.05). All mechanical properties for the experimental groups (e.g., flexural strength, elastic modulus, and microhardness) were significantly lower than those of the control group (<i>p</i> &lt; 0.05). For Sr-PBG concentrations ≤ 5&#xa0;wt%, the mechanical parameters were within the acceptable ranges set by ISO standards. Significant color differences (Δ<i>E</i>₀₀) were observed between the control and experimental groups, with the 10&#xa0;wt% Sr-PBG sample showing clinically unacceptable levels. The release of Sr, P, and Ca ions increased with the Sr-PBG content before thermocycling. After thermocycling, Sr and Ca release decreased, whereas P release increased sharply at concentrations above 5&#xa0;wt%. Antibacterial tests confirmed that higher Sr-PBG concentrations resulted in superior antibacterial efficacy against <i>S. mutans</i>. The Sr-PBG-containing composites were not cytotoxic to human cells. The findings suggest that 5&#xa0;wt% Sr-PBG is the optimal concentration for improving the antibacterial properties of dental restorative materials without compromising their clinical performance.</p>