The left-lateral strike-slip Adiyaman fault is located in eastern Turkey within the plate boundary deformation zone between Arabia and Anatolia. The Adiyaman fault is a major splay from the East Anatolian Fault (EAF), one of the most important tectonic structures in the Eastern Mediterranean region. These faults are consequence of the collision between the Arabian and Anatolian plates and the resulting westward tectonic escape of Anatolia. Although the EAF has been intensively studied since its discovery in the late 1960s, little is known about the Adiyaman fault and its tectonic activity. In this study, we extract geomorphic indices including mountain-front sinuosity (S-mf), valley floor width-to-height ratio (V-f), stream length-gradient (S-L), catchment Asymmetry Factor (AF) and hypsometric integrals and curves (HI and HC) to evaluate the relative tectonic activity of the Adiyaman fault. These three geomorphic indices (AF, HI, and HC) are averaged to define an index for Relative Tectonic Activity (RTA) that allows the Adiyaman fault to be divided into categories of low, moderate and high RTA. The results confirm that the Adiyaman fault is an active fault with high to moderate Quaternary tectonic activity. However, this fault is of minor importance on accommodating plate boundary deformation, as evidenced by the recent crustal motions determined by GPS studies. Nevertheless, it is worthwhile to note that the Adiyaman fault still poses a significant seismic hazard for the region despite its relatively moderate tectonic activity.