Electron spin resonance (ESR) spectroscopy is a crucial tool to determine the chemical structure of materials. ESR spectra measured in molecular systems, however, are established on large ensembles of spins and usually require complicated structural analysis. Recently, scanning tunneling microscopy (STM) combined with ESR has been proven as a powerful tool to image and coherently control individual atomic spins on surfaces. Here, we extend this technique to demonstrate ESR on single organic molecules - iron phthalocyanine (FePc) - and investigate the magnetic interactions between a molecular spin and either another molecular or atomic spin. We show that the molecular spin density is not only localized at the central Fe atom, but also distributed to the outer ligands, yielding a strongly anisotropic exchange coupling. Our work opens the door for using molecules in nanoscale ESR studies and promises tuning magnetic interactions between non-localized spins via tailoring ligand field symmetry and strength.