Retrosternal pericardial adhesions form post-cardiac surgery and are associated with greater mortality risks for patients undergoing re-sternotomies. Fibrous adhesion development is mediated by cardiac myofibroblasts. Expression of proteoglycan 4 (PRG4) in the human pericardial anatomy and the biological functions of recombinant human PRG4 on human cardiac myofibroblasts was investigated. Expression of PRG4 in the human pericardial fluid and localized expression of PRG4 by pericardial mesothelial cells lining the pericardial cavity was observed. Recombinant human PRG4 prevented human cardiac myofibroblast adhesion to coated surfaces, attenuated transforming growth factor-β1 mediated collagen matrix gel contraction, decreased collagen fiber alignment by activated cardiac myofibroblasts and increased cell roundness. Replacing the lost endogenous PRG4 found in pericardial fluid with recombinant human PRG4 could attenuate the formation of post-operative pericardial adhesions and increase the safety and feasibility of re-sternotomies.