ABSTRACT 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 Ld≃ 9 × 1046 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 (>109 M⊙) black holes at high redshifts.