To enhance the breeding of new scab-resistant apple cultivars, a comprehensive understanding of the mechanisms governing major scab resistance genes is essential. Rvi12_Cd5 was previously identified as the best candidate gene for the Rvi12 scab resistance of the crab apple “Hansen's baccata #2” by gene prediction and in silico analysis. In the present study, Rvi12_Cd5 was used to transform the scab-susceptible apple cultivar “Gala Galaxy.” Two constructs were prepared: the first carrying Rvi12_Cd5 under the control of a 35S promoter and E9 terminator, and the second carrying Rvi12_Cd5 under the control of its native promoter and terminator. All the transgenic lines were analyzed for T-DNA integration, copy number, and expression of Rvi12_Cd5 and phenotypically evaluated for scab resistance. The “Gala Galaxy” lines carrying the 35S promoter expressed Rvi12_Cd5 at a high level, showing partial to high resistance against a mixed inoculum of Venturia inaequalis, with symptoms ranging from class 0 to 3b on the Chevalier scale. The transgenic lines carrying the native promoter showed a lower expression of Rvi12_Cd5 compared with the 35S lines. Nevertheless, the low expression was sufficient to induce a resistance level comparable to that of the transgenic lines carrying the 35S promoter. These results indicate that Rvi12_Cd5 confers scab resistance to a susceptible apple cultivar and that even a low level of gene transcript can trigger a plant response to V. inaequalis infection. After HcrVf2 and Vr2-C, Rvi12_Cd5 is the third major apple scab resistance gene being functionally proven.