The aim of this study was to investigate the antimycobacterial activity of 39 free terpenes and their activity in combination with streptomycin. Antimicrobial activity was first evaluated by screening 39 free terpenes at concentrations from 1.56 to 400 µg/mL. None of these exhibited positive effects against any of the nontuberculous mycobacteria (NTM) strains tested. However, six of the 39 terpenes (isoeugenol, nerol, (+)-α-terpineol, (1R)-(−)-myrtenol, (+)-terpinen-4-ol, and eugenol) were shown to enhance the activity of streptomycin against the NTM strains isolated from diseased ornamental fish.