Field studies of the Famennian/Tournaisian (Devonian/Carboniferous, D/C) interval in the Alborz Basin, Iran (Kalariz section) yielded no complete trilobites. However, thin-section analysis presents a different narrative, identifying numerous fragmentary trilobite elements. The fragmentary nature of these remains precludes identification to the genus level. Trilobite sclerites are present in most Upper Devonian carbonate layers but disappear around the D/C boundary and do not reappear until a significant horizon approximately 40 m above it (the Upper part of the Si. isosticha-Upper Si. crenulata to Gnathodus typicus Zone) in the upper Tournaisian, indicating a significant evolutionary gap as well as an important systematic record of Carboniferous trilobites in the Alborz Basin above the D/C black shales. The trilobite exoskeleton comprises a series of articulated segments that readily disarticulate during molting and are most commonly preserved as fragmentary elements. These include hook-like fragments, transverse sections exhibiting axial and lateral regions, individual axial rings, and eyes. We identified a unique assemblage featuring two highly articulated trilobite specimens and a closely-associated isolated cephalon, preserving the cephalon, thorax, pygidium, axial lobe, and ocular remnants. The thoracic pleurae are visible, terminating in circular to oval points. Two specimens are preserved in a linear, end-to-end alignment, while the third consists solely of a cephalon. Two distinct forms of compound eyes are recognized in the trilobites: the common sessile, faceted type and the rarer stalked type, which may exhibit a smooth visual surface.