Tanshinone I (Tanshinone-1), a major active principle of the traditional Chinese medicine Salvia miltiorrhiza, possesses excellent anticancer properties, including inhibiting proliferation, angiogenesis and metastasis and overcoming multidrug resistance (MDR). However, its direct anticancer molecular target(s) remain unknown. Here we report that tanshinone-1 and its two new derivatives, S222 and S439, directly inhibit DNA topoisomerase I/II (Top1/2). With significantly improved water solubility, S222 and S439 displayed 12- and 14-times more potent proliferative inhibition than their parent tanshinone-1 in a panel of 15 cancer cell lines. Both retained tanshinone-1’s anti-MDR and anti-angiogenesis properties and its capability to reduce the phosphorylation of Stat3 at Tyr705 with apparently enhanced efficacy and in these regards, S439 was also slightly more potent than S222. Both derivatives and tanshinone-1 directly inhibited Top1 and Top2 at molecular and cellular levels; the derivatives displayed similar potency but both were more potent than tanshinone-1. The inhibition of S222 and S439 on Top1 and Top2 was also more potent than that of the Top1 inhibitor hydroxylcamptothecin and the Top2 inhibitor etoposide, respectively. Consistently, tanshinone-1 and its derivatives induced DNA double-strand breaks, G2/M arrest and apoptosis. Unexpectedly, the derivatives demonstrated different p53-dependency in inducing both cell cycle arrest and apoptosis. S222 showed no obvious p53-dependency. In contrast, S439 induced more G2/M arrest in p53-proficient cells than in p53-deficient cells while its apoptotic induction was the opposite. However, their proliferative inhibition was independent of the p53 status. Due to their structures different from the known Top1, Top2 and dual Top1/2 inhibitors, our results indicate that tanshinone-1 and its derivatives are a new type of dual Top1/2 inhibitors.