Abstract We present ALMA Band 9 continuum observation of the ultraluminous quasi-stellar object (QSO) SDSS J0100+2802 providing a ∼10σ detection at ∼670 GHz. SDSS J0100+2802 is the brightest QSO with the most massive supermassive black hole (SMBH) known at z > 6, and we study its dust spectral energy distribution in order to determine the dust properties and the star formation rate (SFR) of its host galaxy. We obtain the most accurate estimate so far of the temperature, mass, and emissivity index of the dust, which are T _dust = 48.4 ± 2.3 K, M _dust = (2.29 ± 0.83) × 10⁷ M _⊙, and β = 2.63 ± 0.23, respectively. This allows us to measure the SFR with the smallest statistical error for this QSO, SFR = 265 ± 32 M _⊙yr⁻¹. Our results enable us to evaluate the relative growth of the SMBH and host galaxy of J0100+2802. We find that the SMBH is dominating the process of black-hole galaxy growth in this QSO at z = 6.327, when the universe was 865 Myr old. Such unprecedented constraints on the host-galaxy SFR and dust temperature can only be obtained through high-frequency observations and highlight the importance of ALMA Band 9 to obtain a robust overview of the buildup of the first quasars’ host galaxies at z > 6.