Objectives: Nanoparticles induce oxidative stress in cells and damage them through the cell membrane and DNA damage, eventually resulting in cell death. This study aimed to evaluate the effect of titanium dioxide (TiO2) nanoparticles on apoptosis induction and invasion of gastric cancer cell line, MKN-45. Methods: We used the MTT assay to assess proliferation of MKN-45 gastric cancer cells after exposure to different forms of TiO2 nanoparticles including amorph, brookite, anatase, and rutile coated with polyethylene glycol (PEG) and bovine serum albumin (BSA). Ethidium bromide and acridine orange staining were used to visualize cancer cell apoptosis, and the wound healing assay technique (migration test) was used to assay cancer cell invasion. Results: Viability and proliferation of cancer cells in the presence of various forms of TiO2 nanoparticles were reduced (p ≤ 0.050). This reduction in cell proliferation and viability was directly related to concentration and duration of exposure to nanoparticles. Induction of cell death was seen in all groups (p ≤ 0.050). Increased cell invasion was seen in PEG-amorph TiO2 group compared to the control group. Cell invasion was decreased only in the brookite BSA group (p ≤ 0.050). Conclusions: Various forms of TiO2 nanoparticles reduced cell proliferation and induced apoptosis in cancer cells. Some forms of TiO2 nanoparticles such as brookite BSA also inhibited cell invasion. PEG-amorph TiO2 nanoparticles increased cell invasion. These differences seem to be due to the effects of different configurations of TiO2 nanoparticles. TiO2 may provide a new strategy for cancer treatment and more studies are needed.