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Context: The extreme luminosity of gamma-ray bursts (GRBs) makes thempowerful beacons for studies of the distant Universe. The most luminous burstsare typically detected at moderate/high redshift, where the volume for seeingsuch rare events is maximized and the star-formation activity is greater thanat z = 0. For distant events, not all observations are feasible, such as at TeVenergies.  Aims: Here we present a spectroscopic redshift measurement for theexceptional GRB 221009A, the brightest GRB observed to date with emissionextending well into the TeV regime.  Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope(VLT) to obtain simultaneous optical to near-IR spectroscopy of the burstafterglow 0.5 days after the explosion.  Results: The spectra exhibit both absorption and emission lines from materialin a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burstwith a luminosity distance of 745 Mpc. Its host galaxy properties(star-formation rate and metallicity) are consistent with those of LGRB hostsat low redshift. This redshift measurement yields information on the energy ofthe burst. The inferred isotropic energy release, $E_{\rm iso} > 5 \times10^{54}$ erg, lies at the high end of the distribution, making GRB 221009A oneof the nearest and also most energetic GRBs observed to date. We estimate thatsuch a combination (nearby as well as intrinsically bright) occurs between onceevery few decades to once per millennium.

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151