Polycaprolactone (PCL) is a biodegradable polymer showing excellent promise for application to environmentally sustainable materials. Among various biodegradable polymers, PCL comprises semicrystalline low-melting-point (∼60 °C) aliphatic polyesters, which simplify processing. However, disadvantageous mechanical properties limit the practical applications of PCL. In this study, cellulose nanocrystals (CNCs) and PCL were subjected to in-situ polymerization to synthesize a CNC–PCL nanocomposite with improved mechanical properties compared to those of PCL. Additionally, solvent exchange was used to optimize the hydrophilic-CNC dispersion in the hydrophobic PCL matrix and ε-caprolactone monomer for the ring-opening polymerization. This approach was used to prepare a homogeneously dispersed 0.3 wt% CNC-loaded nanocomposite exhibiting a 1.4-fold-higher ultimate tensile strength of 61 MPa and 1.2-fold-increased elongation at break of 1,340%. Moreover, the PCL/CNC nanocomposite exhibited a tear toughness 1.7-fold higher than that of neat PCL and could broaden the industrial-application range of reinforced bioplastics.