Salicylaldehyde dehydrogenase (sALDH) can oxidize salicylaldehyde, which is an intermediate in the naphthalene catabolism in bacteria. However, genes encoding sALDH have not been discovered so far in Rhodococcus. Here, we report the discovery of a novel aldehyde dehydrogenase (ALDH) gene in the naphthalene degrader Rhodococcus ruber OA1 based on phylogenetic analysis. Interestingly, apart from ALDH activity, ALDH of R. ruber OA1 (OA1-ALDH) also showed sALDH activity. Moreover, its sALDH specific activity was higher than its ALDH specific activity. Based on a comparison with the ALDH of Thermomonospora curvata DSM 43,183, a putative active site Cys123 and NAD(+) binding site Asn263 were proposed in R. ruber OA1. Multiple alignment of OA1-ALDH with ALDHs from other organisms indicated that the residues Ser122 and Ala124 might influence the enzyme activity and substrate specificity that render OA1-ALDH the ability to catalyze salicylaldehyde better than acetaldehyde. These results support the possibility that OA1-ALDH plays the role of sALDH in the oxidation of salicylaldehyde to salicylate in R. ruber OA1. In summary, our study would contribute to the understanding of the structure and roles of ALDH in Rhodococcus.