This study aimed to demonstrate that organic anion transporters (OATs) mediate the drug-drug interaction (DDI) between piperacillin and tazobactam. After co-administration with piperacillin in rats, the AUC of tazobactam in plasma was significantly increased, and t1/2β was prolonged with significant reduction in plasma clearance, renal clearance and cumulative urinary excretion. In rat and human kidney slices, probenecid, p-aminohippurate and benzylpenicillin inhibited the uptake of piperacillin and tazobactam. Piperacillin significantly inhibited the uptake of tazobactam. Moreover, the uptakes of piperacillin, tazobactam and sulbactam in hOAT1/3-HEK293 cells were significantly higher compared with mock-HEK293 cells, respectively. Piperacillin significantly inhibited the uptake of tazobactam in hOAT1/3-HEK293 cells. The Km values of tazobactam (431 ± 67 μM for hOAT1, 377 ± 63 μM for hOAT3) were significantly higher than those of piperacillin (37 ± 5 μM for hOAT1, 172 ± 28 μM for hOAT3). This suggested that piperacillin has a stronger affinity to hOAT1/3 than tazobactam. Meanwhile, the Km values of tazobactam were increased in the presence of piperacillin with unchanged Vmax. This indicated that piperacillin inhibited the uptake of tazobactam in a competitive manner. In conclusion, piperacillin and tazobactam are the substrates of hOAT1/3, and OAT1/3 mediate the DDI between piperacillin and tazobactam.