We report the discovery of the BL Lacertae object FIRST J233153.20+112952.11 at redshift >6 using near-infrared spectroscopy and broadband observations in the infrared, submillimeter, and radio wavelengths. The detection of the continuum break at ∼0.92 μm in the near-infrared spectrum of FIRST J233153.20+112952.11 provides an approximate redshift of 6.57, corresponding to an age of the universe of ∼800 Myr. The rest-frame UV/optical spectrum of FIRST J233153.20+112952.11 shows no emission lines detected with equivalent width >1.5 Å. The nondetection of the C iv λ1549 broad emission line constrains the luminosity of the accretion disk to be ≲ 21045 erg s-1, which corresponds to the radiatively inefficient accretion regime of BL Lacertae objects. The spectral index of the rest-frame UV/optical continuum, α ν,opt = 1.43 ± 0.23, is consistent with the expected spectral index of the synchrotron emission spectrum of the relativistic jet. The flat radio continuum (α ν,r ≈ 0) in a rest-frame frequency interval of 7-23 GHz is similar to that of the typical BL Lacertae objects and also consistent with originating from the synchrotron jet emission. The rest-frame UV/optical and radio fluxes of FIRST J233153.20+112952.11 show significant (15%-40%) variations. The shortest recorded rest-frame timescale of these variations is ∼8 days. Assuming the local scaling relation between the radio luminosity and black hole mass, the 5 GHz radio luminosity of FIRST J233153.20+112952.11, 1.6 × 1033 erg s-1 Hz-1, suggests a billion solar mass central supermassive black hole.