Significant research has focused in recent years on how to integrate CNCs into polymeric materials. The polar and hydrophilic nature of CNCs makes them incompatible with most polymers. Furthermore, the high specific surface area of the nanosized particles promotes aggregation and results in inefficient dispersion and weak interfacial interactions. Chemical modification of the surface of the nanocrystals to improve compatibility in most cases leads to a loss of their inherent properties. Thus, melt compounding methods have been met with limited success. In this work, in situ polymerization is investigated as an efficient method for dispersing CNC in condensation polymer matrices. Poly(ethylene succinate) (PESu) is a biodegradable aliphatic polyester with similar mechanical properties to linear low-density polyethylene. However, low crystallization rates limit its processability. Effective dispersion of CNCs within PESu is investigated here as a way to improve the crystallization properties without detrimentally affecting degradability. The addition of CNCs directly into the PESu polymerization mixture using in situ polymerization dispersed the nanoparticles efficiently, as individual particles, without visible cluster formation. The polymerization kinetics were altered with increasing CNC content, causing a decrease in the polymer molecular weight. The addition of small amounts of CNCs, up to 0.25 wt%, resulted in a slight decrease in viscosity, whereas significant increases in the zero shear viscosity were observed at 1 wt% CNC. PESu containing well-dispersed 0.25 wt% CNC maintained the tensile properties of the matrix, while demonstrating significantly improved crystallization kinetics and reduced haze due to the formation of smaller crystallites. CNC acted as a very efficient nucleating agent, while biodegradability remained unimpeded. Nanocomposites of PESu and CNC with improved crystallization kinetics may provide a biodegradable “green” alternative to plastics commonly used for general purpose biodegradable containers and food packaging.