In this work, we present the first spectroscopic measurements of conversion electrons originating fromthe decay ofmetastable gaseous $^{83m}$Kr with the Karlsruhe Tritium Neutrino (KATRIN) experiment. The obtained results represent one of the major commissioning milestones for the subsequent direct neutrino mass measurement with KATRIN. The successful campaign demonstrates the functionalities of the KATRINbeamline. Precisemeasurement of the narrowK- 32, L$_{3}$-32, and N$_{2,3}$-32 conversion electron lines allowed to verify the eV-scale energy resolution of the KATRIN main spectrometer necessary for competitive measurement of the absolute neutrino mass scale.
