Systemic injection into blood vessels is the most common method of drug administration. However, targeting drugs to the heart is challenging, owing to its dynamic mechanical motions and large cardiac output. Here, we show that the modification of protein and peptide therapeutics with tannic acid-a flavonoid found in plants that adheres to extracellular matrices, elastins and collagens-improves their ability to specifically target heart tissue. Tannic-acid-modified (TANNylated) proteins do not adsorb on endothelial glycocalyx layers in blood vessels, yet they penetrate the endothelium to thermodynamically bind to myocardium extracellular matrix before being internalized by myoblasts. In a rat model of myocardial ischaemia-reperfusion injury, TANNylated basic fibroblast growth factor significantly reduced infarct size and increased cardiac function. TANNylation of systemically injected therapeutic proteins, peptides or viruses may enhance the treatment of heart diseases.