The purpose of this study was to examine the effects of catechins and their derivatives on the activities of Arg‐gingipain (Rgp) and Lys‐gingipain (Kgp) in Porphyromonas gingivalis. Catechin derivatives, which included (−)‐epigallocatechin gallate, (−)‐epicatechin gallate, (−)‐gallocatechin gallate, and (−)‐catechin gallate, significantly inhibited the Rgp activity. The 50% inhibitory concentrations (IC50s) of these catechin derivatives for Rgp ranged from 3 to 5 μm. While (−)‐epigallocatechin and (−)‐gallocatechin moderately inhibited Rgp activity (IC50s, 20 μm), (−) ‐epicatechin, (+)‐catechin, and gallic acid were not effective, with IC50s greater than 300 μm. Further, some of the catechin derivatives tested also inhibited the Kgp activity, though to a lesser extent than inhibition of the Rgp activity. These findings suggest that green tea catechins may have the potential to reduce periodontal breakdown resulting from the potent proteinase activity of P. gingivalis.