Russellysin (, Russell's viper venom factor X activator, RVV-X, blood-coagulation factor X activating enzyme, metalloproteinase RVV-x, Vipera russelli proteinase, Russell's viper blood coagulation factor X activator, RVV-V) is an enzyme. This enzyme catalyses the following chemical reaction
This enzyme is present in the venom of Russell's viper (Vipera russelli).