A new amphipod species belonging to the genus Gammarus was identified in the rivers of the Eastern Black Sea Region of Türkiye: G. sezginisp. nov. The authors described the new species using a taxonomic approach that combines morphological and molecular data. The newly identified species belongs to the G. komareki species complex because of the setation of antenna 2, pereopods 3 and 4, and the uropod 3. Some of its characteristic features are as follows: A medium-large species (holotype male, 9.8 mm). The body is yellowish; no dorsal keel or hump; eyes well developed, kidney-shaped; extremities not elongated; the second antenna bears numerous groups of long setae on the peduncle and flagellar segments; antennal gland cone long, not curved; the posterior margin of pereopod 3 is densely setose; the setae on the posterior edge of pereopod 4 are shorter and fewer in number; the anterior margins of pereopods 5 to 7 bear spines in the male; epimeral plates are not pointed. The newly identified species looks similar to G. komareki but differs from it by having a longer antennal gland cone, having fewer D-setae (33) in the third segment of the mandible palp, having shorter setae on the ventral part of the peduncular segment of the antenna 2, and having longer antenna 1, having fewer setae along the posterior margins of pereopods 3 and 4, and the absence of setae along the anterior margins of merus and carpus of pereopod 7. The new species is distinct from its relatives by high genetic distance (COI: 17.10% and 28S: 0.88%) and was resolved from them as an independent lineage with high support (ML: 78%, NJ: 70%, and BI: 1.0) in all phylogenetic results, based on the concatenated dataset (28S+COI). Additionally, species delimitation analyses (ASAP and PTP) based on the COI gene supported the conclusion that the new species constitutes an independent lineage. Detailed descriptions and drawings of the male holotype and the female allotype are given, and the morphology of the newly identified species is compared with that of its relatives.