Purpose: This study was carried out to evaluate the effect of biostimulation on the regenerative response of immature teeth with necrotic pulp and apical periodontitis in dogs. Materials and Methods: Seventy-two root canals were employed in the present study as follows: Sixty root canals for experimental groups and twelve as positive and negative controls. After the induction of pulp necrosis and apical periodontitis, roots were divided into two groups, group I: Regenerative Endodontic Therapy (RET) with subsequent biostimulation (seven sessions at 808 nm diode laser at output power of
300 mW for 90 seconds) and group II: RET without biostimulation. The groups were followed up for 1, 2 and 3 months (subgroups A, B&C). The maturation of the roots was assessed both radiographically and histologically. All data were statistically analyzed. Results: the application of biostimulation in group I demonstrated marked increase in root length, thickness and decrease in apical diameter compared to group II however, it was statistically significant only in subgroup C (12.43% versus 7.66%, 33.09% versus 17.81% and 38.12% versus 23.35 % at third month) respectively (p < 0.05). Furthermore, it showed histologically the highest score of vital tissue infiltration and least inflammatory scores which was statistically significant. Conclusion: Biostimulation enhanced the response of immature teeth with necrotic pulp and apical periodontitis to regenerative endodontic therapy improving root maturation.