By exposing tetraphenyl-porphyrin (H2-TPP) molecules anchored on a Ag(111) surface to a beam of cerium atoms under vacuum conditions, a selective interaction of the lanthanide with the porphyrin macrocycle is achieved. A novel Ce-TPP species is formed that can be clearly identified in scanning tunneling microscopy data resolving intramolecular features. The electronic structure of the Ce-TPP was characterized by scanning tunneling spectroscopy, identifying molecular resonances that are related to those of coadsorbed Co-TPP. The inhomogenous local electronic density distribution of the dominating Ce-TPP and Co-TPP occupied states was visualized by tunneling spectroscopy mapping. With complementary density functional theory calculations, the geometry and energetics of the Ce-porphyrin bonding was determined.