Herein, an electrochemical sensor based 5-(2-phenylbenzo [b] thiophen-3-yl)furan-2-carbaldehyde (PTFC) was developed for the detection of cancer antigen 125 (CA125). Measurements were obtained via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV), and square wave voltammetry (SWV) techniques to investigate features of the electrochemical sensor such as concentration effect of CA125, incubation time, scan rate, limit of quantification (LOQ), limit of detection (LOD), and interference effect of structures found in serum. The sensor was found to have a fairly wide linear range as 0.01-100 ng mL(-1) and 100-1000 ng mL(-1). LOQ and LOD values were determined as 0.024015 ng mL(-1) and 0.008005 ng mL(-1) (S/N = 3), respectively. These results showed that the PTFC-based electrode could be a promising electrode for the detection of CA125.