The anion-pi bond has emerged as an important nonvalence interaction in supramolecular and biological structure. Although recognized as a strong noncovalent interaction, driven by electrostatic charge-quadrupole moment and correlation interactions, benchmark experimental and computational studies on the intrinsic anion-pi bond strength are scarce. Here, we present a gas-phase photoelectron spectroscopic study on the archetypical iodide-hexafluorobenzene anion-pi bonded complex. In combination with high-level electronic structure calculations, the anion-pi bond strength is found to be 0.53 eV (51 kJ mol(-1)). The interaction arises for a large part from correlation forces (similar to 40%), with electrostatic quadrupole-anion and polarization making up most of the remainder.