A valve stem seal exhibits superior workability of mounting of a reinforcing ring to a valve stem guide and enables the reinforcing ring to be fixed to the valve stem guide in a stable manner. A reinforcing ring is mounted to a valve stem guide by fitting a cylindrical portion to the valve stem guide from a side opposite to an inward flange portion. The cylindrical portion includes a slit extending linearly in a circumferential direction and a locking protrusion that is provided on a side opposite to the inward flange portion with respect to the slit and that is to be locked by a locking recess formed in the valve stem guide. The locking protrusion is constituted by a curved portion that is gradually recessed radially inward from the side opposite to the inward flange portion with respect to the slit to a position that reaches the slit.