This work aimed at developing a methodology for examining the naturally grown passive layer of a thickness of just a few nanometers of an austenitic CrMnN steel by means of atom probe tomography and gaining knowledge on the structure of this alloy's passive layer. The sample surface was ground, polished, cleaned, degreased, electrolytically polished, and oxidized in air to produce a reproducible passive layer. The oxide layer was subsequently coated with a silver layer of a thickness of 3 μm. The silver layer protects the oxide layer during the preparation of the atom probe tips in the focused ion beam microscope and the alignment of the tip in the atom probe. The samples were measured in the atom probe's pulsed-voltage mode. The findings show that an enrichment of oxygen, molybdenum, nitrogen, and chromium and a depletion of manganese, nickel, and iron occur in the area of the passive layer.