The intermetallic compound Mn5Ge3 is one of the promising materials for application as a source of charge carriers in spintronics. The existing experimental data on the spin polarization in Mn5Ge3 demonstrate significant discrepancies. All theoretical studies concern a Mn5Ge3 bulk crystal. At the same time, thin films are of interest for applications. In this work, ab initio calculations have been performed for a Mn5Ge3 thin film on a germanium substrate. The difference between the magnetic moments of manganese atoms, densities of states, and spin polarizations for the bulk crystal and thin film has been demonstrated.