A356 with scandium (Sc) addition provides interesting results beyond costs. For the practical use of Sc, the effects of Sc on castability must be considered. Fluidity and hot cracking are important factors defining the castability of aluminum casting alloys. In the present work, the influence of Sc addition on the castability of A356 hypoeutectic Al–Si alloy was investigated, which was evaluated through fluidity and hot cracking susceptibility. The fluidity of the alloys was studied by measuring the total volume of solidified aluminum in a multi-channel mold. The hot cracking susceptibility of the alloys was evaluated by using a constrained-rod casting mold test. The results of the fluidity and hot cracking susceptibility test were supported by microstructural analysis. The results indicate that 0.2 wt% Sc addition significantly increases the fluidity of A356 alloy, due to the grain refinement and eutectic Si modification by changing the solidification mode. However, the fluidity slightly decreases when the Sc content increases to 0.4 wt% due to the formation of primary Al2Si2Sc intermetallic phase. The hot cracking of A356 alloy was completely diminished when Sc was added to the alloy.