Nowadays, nanomaterials that are smaller than 100nm in size are very attractive owing to their enhanced physicochemical properties. Although they have been used widely for industrial applications, their toxicity still remains a problem. This article is a new record of the synthesis of titanium dioxide nanoparticles (TiO2 NPs) by a Mentha aquatica leaf extract and determination of its toxicity to rat marrow mesenchymal stem cells. In this study, we aimed to determine the genotoxic and cytotoxic effects of biologically synthetized TiO2 NPs. The characteristic peak of the nanomaterial was observed at 354nm. The mean size of the nanomaterial was measured to be 69nm from SEM images. According to zeta analysis, the surface charge of the nanomaterial was -37.6mV. The crystalline structure of the nanomaterial was determined using XRD analysis. It was concluded that the obtained nanomaterial was TiO2 The results of the FT-IR analysis showed that the functional groups that were found in the plant extract could play an important role in the formation and stabilization of TiO2 NPs. The effective size of the TiO2 NPs was found to be 304nm using DLS analysis. The TGA analysis results showed that the total mass loss was 4% at 900 degrees C. According to DNA cleavage analysis results, TiO2 NPs cause damage to the plasmid pBR322 DNA in a concentration-dependant matter. It has been noted that TiO2 NPs lead to decreased cell viability during increased time and concentration of applications on rat marrow mesenchymal stem cells. It has also been determined that bulk TiO2 causes a greater reduction in the stem cell viability compared to the biosynthesized NPs. The obtained results could be useful for further application and toxicity studies.