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.