A chemical method was used to prepare Cd 1−x Ni x Te 0.5 Se 0.5 (Cd 1−x Ni x TeSe, x = 0–0.1) quantum dots (QDs) with particle sizes of 3–4 nm. Structural analyses of x-ray diffraction patterns indicate that all QDs are single-phase and crystallize in the zincblende-type structure. The lattice constant gradually decreases with increasing x in Cd 1−x Ni x TeSe. This is due to a partial replacement of Ni (a smaller ion) for Cd 2+ (a larger ion). Our study also indicates that the Ni doping causes the red shift of the longitudinal optical mode, the blue shift of the excitonic absorption edge and photoluminescence (PL) peak, and a gradual decrease of the PL quantum yield. When the excitation power increases, the PL peak of CdTeSe (x = 0) is almost unchanged, while that of Cd 1−x Ni x TeSe QDs (x > 0) shifts linearly towards high energies, which is related to the state-filling effect caused by Ni 2+ dopants. Comparing with pure CdTeSe, Ni-doped QDs have longer PL decay times, up to ∼ 580 ns. Particularly, all QDs exhibit weak ferromagnetic (FM) order at room temperature generated from defect-mediated exchange interactions of Ni 2+ ions. Such results proved ternary Cd 1−x Ni x TeSe QDs having simultaneously the optical and FM properties. Together with very long decay times, they are considered as potential materials for biosensing, photovoltaic and photocatalytic applications.