In this study, the effect of green microalgae as a nucleating agent on the non-isothermal crystallization of poly(lactic acid) (PLA) was investigated by differential scanning calorimeter and polarized optical microscope. The parameters of non-isothermal crystallization kinetics of PLA and PLA/green microalgae composites were obtained through common kinetic models such as Avrami, Ozawa, and combined Avrami and Ozawa. Moreover, nucleating activity of green microalgae and activation energies of composites were determined by the Dobreva-Gutzow and the Friedman methods, respectively. Dispersion of green microalgae in PLA was characterized using scanning electron microscopy. PLA/green microalgae composites having different green microalgae content from 5 to 20 wt% were prepared via solvent casting method. Although some agglomerated green microalgae was observed in the PLA matrix, it was found that the green microalgae were homogeneously distributed within the matrix at its lower loading levels. The Avrami and combined Avrami-Ozawa results showed that PLA/green microalgae composites have faster melt crystallization rates than that of pure PLA. Nucleating activity of green microalgae was proved by the Dobreva and Gutzow kinetic approach. Moreover, polarized optical microscope analysis revealed that crystallization onset temperature of PLA shifted to higher temperature upon cooling in the presence of green microalgae.