Phenothiazine (PTZ) derivatives have been acknowledged as versatile compounds with significant implications across various areas of medicine, particularly, in cancer research. The cytotoxic effects of synthesized compounds on both normal and cancerous cells, along with their oxidant–antioxidant properties, are pivotal factors in cancer treatment strategies. In the current study, eight new PTZ derivatives were synthesized and the compounds' cytotoxic activities were assessed by 3‐[4,5‐dimethylthiazol‐2‐yl]‐2,5 diphenyl tetrazolium bromide (MTT) assay while the oxidant–antioxidant properties were evaluated by oxidative stress index (OSI) calculation in SH‐SY5Y (a human neuroblastoma cell line), HT‐29 (a human colorectal adenocarcinoma cell line), and PCS‐201‐012 (a human primary dermal fibroblast cell line) cells. Consequently, the half‐maximal inhibitory concentration (IC50) values of compound 3a were determined to be 218.72, 202.85, and 227.86 μM while the IC50 values of compound 3b were defined to be 227.42, 199.27, and 250.11 μM in PCS‐201‐012, HT‐29, and SH‐SY5Y cells, respectively. Additionally, it was determined that the synthesized compounds demonstrated the lowest OSI in PCS‐201‐012 cells as compared to the other cell lines.