Four diterpenoid tanshinones and three phenolic acids were isolated from the crude ethanol extract of the cultured hairy roots of Salvia miltiorrhiza Bunge by bioassay-guided fractionation. By means of physicochemical and spectrometric analysis, they were identified as tanshinone ΙΙA (1), tanshinone Ι (2), cryptotanshinone (3), dihydrotanshinone Ι (4), rosmarinic acid (5), caffeic acid (6), and danshensu (7). These compounds were evaluated to show a broad antimicrobial spectrum of activity on test microorganisms including eight bacterial and one fungal species. Among the four tanshinones, cryptotanshinone (3) and dihydrotanshinone Ι (4) exhibited stronger antimicrobial activity than tanshinone ΙΙA (1) and tanshinone Ι (2). The results indicated that the major portion of the antimicrobial activity was due to the presence of tanshinones and phenolic acids in S. miltiorrhiza hairy roots, which could be used as the materials for producing antimicrobial agents for use in agricultural practice in the future.