Due to their distinctive electronic, mechanical, thermal, and optoelectronic properties, two-dimensional (2D) materials have emerged as valuable alternatives for thermoelectric (TE) materials. Recently, graphitic carbon nitride (g-C3N4), a metal-free semiconductor, has attracted attention in many applications, including energy conversion and storage. However, research on the use of g-C3N4 in TE applications is still limited. In this study, the TE properties of exfoliated g-C3N4-filled unsubstituted polythiophene (PTh) composites were determined for the first time. For this purpose, g-C3N4 was synthesized from guanidine-hydrochloride by thermal treatment, and the exfoliation process was carried out by ultrasonic homogenization method. The composites containing different weight ratios of g-C3N4 were prepared. The peak shifts observed from FTIR-ATR and UV-vis analyses indicated the electrostatic interactions between g-C3N4 and PTh. From the electrical conductivity measurements of the samples, it was obtained that the electrical conductivity of the pristine PTh increased from 1.3x10−3 Scm−1 to 100.1 Scm−1 whereas the Seebeck coefficient increased from 9.5 µVK−1 to 116 µVK−1. Additionally, the power factor of the pristine PTh increased from 1.28x10−5 µWm−1K−2 to 85.81 µWm−1K−2. Based on the results, this study suggests that g-C3N4 is an efficient material to be used as an additive to enhance the TE properties of PTh.