Co-doped CdSSe alloy quantum dots (QDs) were successfully synthesized via a wet chemical hot-injection method, with Co²⁺ doping concentrations ranging from 1 to 10% at. In addition to the intrinsic band gap tunability of CdSSe QDs, Co incorporation introduces magnetic functionality, enabling the development of diluted magnetic semiconductor nanostructures for optoelectronic and spintronic applications. The effects of sulfur/selenium ratio and cobalt doping on the structural, optical, photoluminescence, and magnetic properties of CdSSe QDs were systematically investigated. X-ray diffraction results confirm that both undoped and Co-doped CdSSe QDs crystallize in the cubic zinc-blende structure, with no secondary phases detected. The lattice constant decreases with increasing Co concentration due to the substitution of smaller Co²⁺ ions for Cd²⁺ ions in the host lattice. Optical absorption and photoluminescence measurements reveal that the emission wavelength of CdSₓSe1-x QDs can be effectively tuned across the visible region by adjusting the S/Se ratio. Upon Co²⁺ doping, a pronounced blue shift of both absorption and photoluminescence peaks is observed, accompanied by an increase in band gap energy, indicating strong modification of the electronic structure induced by Co-related energy levels. This behavior is attributed to the substitution of Cd²⁺ by smaller Co²⁺ ions, which induces compressive lattice strain and shifts the conduction band edge to higher energy. Time-resolved photoluminescence analysis shows a decrease in carrier lifetime with increasing Co concentration, attributed to enhanced non-radiative recombination via Co²⁺-induced trap states. Magnetic measurements demonstrate that Co-doped CdSSe QDs exhibit weak room-temperature ferromagnetism coexisting with diamagnetic behavior, with saturation magnetization increasing up to 5% Co doping and decreasing at higher concentrations due to the onset of antiferromagnetic interactions. These results demonstrate that Co doping is an effective method for simultaneously tuning the optical and magnetic properties of CdSSe QDs, making them promising candidates for optoelectronic and spintronic applications.