Two phase transitions of K3Fe5F15, at about 290 and 120 K, have been identified by polarized light microscopy and magnetic measurements on a single crystal grown from high temp. soln. with subsequent quenching from 973 K. The former one at about 290 K is a new second-order structural transition from the orthorhombic to a monoclinic phase, as shown by the appearance of new types of ferroelastic domain walls and a rotation of the optical indicatrix around the pseudo-tetragonal principal axis. The transition at about 120 K is second order and magnetic nature, accompanied by anomalies of spontaneous linear birefringence and antiferromagnetic type magnetic susceptibility. The temp. dependence of the magnetization suggests that a spontaneous ferromagnetic component appears in the magnetically ordered phase below 120 K.