Abstract We simultaneously and successfully fit the multiepoch X-ray spectra of the tidal disruption event (TDE) 3XMM J215022.4−055108 using a modified version of our relativistic slim disk model that now accounts for angular momentum losses from radiation. We explore the effects of different disk properties and of uncertainties in the spectral hardening factor f _c and redshift z on the estimation of the black hole mass M _• and spin a _•. Across all choices of theoretical priors, we constrain M _• to less than 2.2 × 10⁴ M _⊙ at 1σ confidence. Assuming that the TDE host is a star cluster associated with the adjacent, brighter, barred lenticular galaxy at z = 0.055, we constrain M _• and a _• to be 1.75 − 0.05 + 0.45 × 10 4 M _⊙ and 0.8 − 0.02 + 0.12 , respectively, at 1σ confidence. The high, but sub-extremal, spin suggests that, if this intermediate-mass black hole (IMBH) has grown significantly since formation, it has acquired its last e-fold in mass in a way incompatible with both the “standard” and “chaotic” limits of gas accretion. Ours is the first clear IMBH with a spin measurement. As such, this object represents a novel laboratory for astroparticle physics; its M _• and a _• place tight limits on the existence of ultralight bosons, ruling out those with masses from ∼10⁻¹⁵ to 10⁻¹⁶ eV.