A novel biocomposite of nanosized calcium silicate (n-CS) and poly(epsilon-caprolactone) (PCL) was successfully fabricated directly using n-CS slurry, not dried n-CS powder, in a solvent-casting method. The in vitro bioactivity of the composite was evaluated by investigating the apatite-forming ability in simulated body fluid. A proliferation assay with mouse L929 fibroblasts was used to test the in vitro biocompatibility. The composition, hydrophilicity, and mechanical properties were also evaluated. Results suggest that the incorporation of n-CS could significantly improve the hydrophilicity, compressive strength, and elastic modulus of n-CS/PCL composites, with the enhancements mainly dependent on n-CS content. The n-CS/PCL composites exhibit excellent in vitro bioactivity, with surface apatite formation for 40% (w/w) n-CS (C40) exceeding that of 20% (w/w) n-CS (C20) at 7 and 14 days. The Ca/P ratios of apatite formed on C20 and C40 surfaces were 1.58 and 1.61, respectively, indicating nonstoichiometric apatite with defective structure. Composites demonstrated significantly better cell attachment and proliferation than that of PCL alone, with C40 demonstrating the best bioactivity. The apatite layers that formed on the composite surfaces facilitated cell attachment (4 h) and proliferation during the early stages (1 and 4 days). Collectively, these results suggest that the incorporation of n-CS produces biocomposites with enhanced bioactivity and biocompatibility.