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Aims. The first relativistic solar proton event of solar cycle 25 (SC25) wasdetected on 28 October 2021 by neutron monitors (NMs) on the ground andparticle detectors onboard spacecraft in the near-Earth space. This is thefirst ground level enhancement (GLE) of the current cycle. A detailedreconstruction of the NM response together with the identification of the solareruption that generated these particles is investigated based on in-situ andremote-sensing measurements. Methods. In-situ proton observations from a fewMeV to $\sim$500 MeV were combined with the detection of a solar flare in softX-rays (SXRs), a coronal mass ejection (CME), radio bursts and extremeultraviolet (EUV) observations to identify the solar origin of the GLE. Timinganalysis was performed and a relation to the solar sources was outlined.Results. GLE73 reached a maximum particle rigidity of $\sim$2.4 GV and isassociated with type III, type II, type IV radio bursts and an EUV wave. Adiversity of time profiles recorded by NMs was observed. This points to ananisotropic nature of the event. The peak flux at E$>$10 MeV was only $\sim$30pfu and remained at this level for several days. The release time of $\geq$1 GVparticles was found to be $\sim$15:40 UT. GLE73 had a moderately hard rigidityspectrum at very high energies ($\gamma$ $\sim$5.5). Comparison of GLE73 toprevious GLEs with similar solar drivers is performed

The First Ground Level Enhancement of Solar Cycle 25 on 28 October 2021