COVID-19 crisis affects ovarium cancer patients seriously. Thus, it is vital to diagnose ovarium cancer, one of the most common types of cancer diagnosed and the causes of death of women around the world, at early stages. Herein, 5-(2-phenylbenzo[b]thiophen-3-yl) thiophene-2-carbaldehyde (PTTC)-based sensor is designed to detect CA-125 more precisely and rapidly via electrochemical methods. PTTC, novel benzothiophene derivative, is synthesized by electrophilic cyclization reactions and Pd-catalyst coupling reactions. Then, PTTC is dispersed homogeneously in Nafion solution, and an ink is obtained. This ink is transferred onto the glassy carbon electrode and CA-125 is incubated on this electrode. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy measurements are employed to determine sensitivity and reliability of CA-125 on PTTC based GCE electrode. The effect of CA 125 concentration, incubation time, scan rate studies are performed by CV to determine the optimum conditions. Optimum conditions are found as 3 mu L PTTC loading, 5000 ng/mL CA125 antigen concentration and 30 min incubation time. Linear range of the PTTC based GCE electrode prepared at optimum conditions are obtained by DPV as 1-100 ng/ml.Limit of detection and limit of quantification values were obtained as 0.0096 ng/mL and 0.0288 ng/mL, respectively. Interference and artificial serum results reveal that this electrode is a promising electrode for CA125 antigen determination for the ovarium cancer. PTTC is a novel and unique material for the detection of ovarium cancer antigen CA-125 and promising for CA-125 antigen detection.