The aim of this note is to describe the computation of post-Minkowskian Hamiltonians in modified theories of gravity. Exploiting a recent relation between scattering amplitudes of massive scalars and potentials for relativistic point-particles we derive a contribution to post-Minkowskian Hamiltonians at second order in the Newton’s constant coming from R3 modifications in General Relativity. Using this result we calculate the associated contribution to the scattering angle for binary black holes at second post-Minkowskian order, showing agreement in the non-relativistic limit with previous results for the bending angle of a massless particle around a static massive source in R3 theories.