Here we present a dataset of in-situ sediment temperatures of gravity core GeoB21363-1 (GC21) recovered during R/V Poseidon cruise POS499 from the summit of Sartori Mud Volcano (Calabrian Arc, Mediterranean Sea). In-situ temperature was measured using Miniaturized Temperature Data Loggers which were mounted on the core barrel of the gravity corer. In total, five loggers were used and measured temperatures in an interval of 0.5 up to 1 m. The gravity corer remained in the seabed for at least 10 minutes to obtain undisturbed temperature conditions of the seabed. Logger specific serial numbers are listed, penetration depth within the seafloor is given in meter (m) and in-situ temperature is given in Celsius (°C).