Genetic and genomic research has greatly advanced our understanding of heart disease; yet a comprehensive map of the protein landscape of living human hearts is still lacking. Here we set out to identify the molecular basis of functional differences between human cardiac chambers by comprehensive protein expression quantification from samples collected in vivo by high-resolution mass spectrometry. Cardiac biopsies of right atria (RA), left atria (LA) and left ventricle (LV) were obtained from seven humans undergoing open chest surgery and analyzed by high-resolution mass spectrometry. We identify hundreds of proteins with a chamber specific expression pattern, supporting the different functional roles of the cardiac chambers, enabling identification of chamber specific drug targets, and offering novel links between genomic data and the mechanisms of disease.