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The radio‐loud quasar SDSS J102623.61+254259.5, at a redshift  z  = 5.3, is one of the most distant radio‐loud objects. Since its radio flux exceeds 100 mJy at a few GHz, it is also one of the most powerful radio‐loud sources. We propose that this source is a blazar, i.e. we are seeing its jet at a small viewing angle. This claim is based on the spectral energy distribution of this source, and especially on its strong and hard X‐ray spectrum, as seen by  Swift , very typical of powerful blazars. Observations by the Gamma‐Ray Burst Optical/Near‐Infrared Detector (GROND) and by the  Wide‐field Infrared Survey Explorer  ( WISE ) allow us to establish the thermal nature of the emission in the near‐IR–optical band. Assuming that this is produced by a standard accretion disc, we derive that it emits a luminosity of  L  d ≃ 9 × 10 46  erg s −1  and that the black hole has a mass between 2 and 5 billion solar masses. This poses interesting constraints on the mass function of heavy (&gt;10 9  M ⊙ ) black holes at high redshifts.

SDSS J102623.61+254259.5: the second most distant blazar at z = 5.3