The aim of the present study was to evaluate the in-vitro cytotoxicity of heat-cure denture base resin (PMMA) modified with recycled denture base resin at 10%, 20%, 30%, 40%, and 50% (w/w) concentration. A total of 30 disk-shaped specimens were prepared and divided into six groups (n = 5). The Control group (R0) consisted of unmodified processed denture base resin, the experimental group consisted of denture base resin processed with substitution of 10% 20%, 30%, 40%, and 50% (w/w) of recycled denture base resin (R10, R20, R30, R40, and R50). Eluates were prepared using five sterile specimens of each group. The mouse fibroblast cell line (L929) was seeded in a 96-well cell plate system at a concentration of 1 × 104 cells/well in the DMEM medium with 1× antibiotic and antimycotic solution and 10% fetal bovine serum at 37°C with 5% CO2 and incubated in a CO2 incubator for 48 h. MTT assay was applied and the absorbance was measured at 570 nm using a microplate reader to assess the in-vitro cytotoxicity. One-way analysis of variance (ANOVA) along with post hoc Scheffe test was used to statistically compare the mean optical density (OD) values and cell survival/viability % amongst the groups. No statistically significant difference was observed in the mean and standard deviation of the optical density and cell viability % of the test groups that were compared. Modification of denture base resin using recycled PMMA does not have a cytotoxic effect on the mouse fibroblast cell line L929.