In aqueous solutions, the helical twist of DNA decreases with temperature. This phenomenon was noticed and studied experimentally several decades ago, but its physical origin remains elusive. The present paper shows that the thermal untwisting can be predicted from the specific properties of the torsional elasticity of the double helix revealed in recent computational studies. The temperature coefficient of untwisting estimated using coarse-grained models fitted to all-atom MD data accounts for the experimental results nearly quantitatively. The agreement is further improved with the computed torsional rigidity scaled to remove the discrepancy from experiment. The results confirm that the torsional rigidity of DNA is strongly anharmonic. They indicate that for random DNA, its value grows with small twisting and decreases with untwisting.