The red photoluminescence of Mn dopants in MnS-CdS heteronanostructures has been observed for the first time. The red photoluminescence at 650 nm derives from emission due to the (4)T(1) --> (6)A(1) transition of Mn(2+) dopants in a CdS matrix exposed to gigapascal-level lattice stress. HRTEM, FFT, XRD, and optical studies revealed that the lattice of Mn-doped CdS is compressed to match that of MnS when CdS crystallizes at the MnS surface to form MnS-CdS heteronanostructures. The photoluminescence decay times of such Mn dopants are on the order of nanoseconds because of the spin-flip interactions between Mn dopants and free carriers in the CdS matrix.