Pt-graphene contacts are fabricated by atomic layer deposition (ALD) on H2 and O2 plasma functionalized graphene. The plasma functionalization of graphene enables the growth of uniform Pt layers on graphene by ALD and improves the Pt-graphene interaction, which results in a reduced Pt-graphene contact resistance. Devices created using a H2 plasma treatment before Pt ALD perform considerably better than the ones created using O2 plasma treatments (Rc = 0.4 ± 0.2 kΩ μm and Rc = 1.2 ± 0.1 kΩ μm, respectively). The lower performance of the O2 plasma treatments is attributed to a less favorable Pt-graphene oxide interaction and the amorphization of graphene due to the O2 plasma interaction. Supporting the latter, density functional theory calculations indicate that the C-H groups created by a H2 plasma treatment leave the π-conjugation (by C-pz orbitals) of graphene largely intact, resulting in good in-plane and out-of-plane conductivity. The C-O groups formed by an O2 plasma treatment however disturb the C-pz character, deteriorating the in-plane conduction of graphene, despite the good out-of-plane conduction. The results indicate that the treatment of graphene by H2 plasma is a straightforward approach to improve the ALD growth on graphene and reduce the Pt-graphene contact resistance.