In the crystal structure of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)mono(bathophenanthroline)erbium(III), C57H73ErN2O6, the Er3+ ion is in an antiprismatic environment, surrounded by two N atoms and six O atoms. The β-diketonato ligands show structural disorder. The ac susceptibility studies conducted at frequencies from 33 to 9995 Hz and at temperatures from 1.7 to 10 K revealed that the application of a static magnetic field induces a slow magnetic relaxation. Two relaxation modes (fast relaxation = FR; slow relaxation = SR) could be distinguished with energy barriers UFR/kB = 15.6 K and USR/kB = 22.4 K and relaxation times of 10–6 and 10–4 s, respectively. The Er3+ compound is luminescent in the solid state, emitting in the near IR region.