Protein Z is vitamin K-dependent member of the coagulation cascade and its functionality is therefore impaired in warfarin therapy. It is a glycoprotein.

The main role of protein Z appears to be the degradation of factor Xa, which is performed by protein Z-related protease inhibitor (ZPI). In some studies, protein Z deficiency has been associated with a propensity to thrombosis, while others link it to bleeding tendency.