A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal 37Ar source was performed. This calibration source features a 35-day half-life and provides two monoenergetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keVare measured to be (32.3±0.3) photons/keV and (40.6 ± 0.5) electrons/keV, respectively, in agreement with oth...