CO2-induced crystallization in semicrystalline polymers commonly occurs in foaming processes. This phenomenon prominently affects the cellular morphology, and hence, properties of the final products. Polylactide (PLA) is a bio-based semicrystalline polymer with commercial importance in medicine and plastics industries. In this work, the impact of dissolved carbon dioxide (CO2) on the isothermal crystallization of linear and long-chain branched (LCB) PLA was investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). PLA with different degree of LCB were produced by the incorporation of a multi-functional chain extender. LCB-PLAs exhibited significantly increased melt viscosity, elasticity, and longer melt relaxation time. Linear and LCB-PLAs were subjected to CO2 at pressure of 0.1-6 MPa and a saturation temperature varying from 25 to 50 °C. The exposure time to CO2 was set in the range of 0.1-6 h. The DSC results indicated that the dissolved gas depressed the glass transition temperature (Tg), hence resulting in a well-known plasticization effect, and increased the degree of crystallinity. The crystallization enthalpy was found to increase with increasing exposure time and saturation pressure due to increased dissolved gas in PLA. The impact of CO2 on the crystallization behavior was less pronounced as the saturation temperature increased. Consistent with DSC results, XRD analysis revealed that the characteristic crystalline peak, corresponding to the α crystalline phase, progressively increased in intensity with increasing exposure time and saturation pressure. Additionally, increase of the exposure time slightly shifted this peak to a lower angle, indicating the reorganization of the crystalline structure. The crystallization process was influenced by the molecular structure and degree of branching. LCB-PLA exhibited a broader characteristic peak in XRD pattern, suggesting the formation of a less perfect crystalline structure.