Fracture toughness is one of the key parameters in evaluating the mechanical behavior of cementitious, rock, and quasi‑rock materials, particularly when these materials are subjected to temperature variations. Given that many engineering structures, such as tunnels and underground facilities, operate across a wide range of temperatures, the simultaneous investigation of temperature effects and specimen geometry on fracture behavior is of particular importance. In this study, the effects of temperature and specimen geometry on the mode‑II fracture toughness of cement mortar were experimentally examined. To this end, tests were conducted on three different specimen geometries, including the centrally notched Brazilian disc, the modified semi‑Brazilian disc with an initial notch, and the Brazilian disc with double diagonal cracks, under four temperature levels of −10, 25, 50, and 100 °C. In addition to fracture toughness, uniaxial compressive strength and the velocities of compressional and shear waves were measured to evaluate changes in internal material quality under thermal effects. The results indicate that decreasing the temperature relative to the reference level improves the internal quality of the specimens and increases fracture toughness, whereas increasing the temperature leads to quality degradation, reduced ultrasonic wave velocities, and ultimately a lower resistance to crack propagation. Moreover, comparison of the geometries revealed that the sensitivity of fracture toughness to temperature variations is governed by the stress distribution and crack‑tip conditions specific to each geometry.