On May 21, 2019 at 03:02:29 UTC Advanced LIGO and Advanced Virgo observed a short duration gravitational-wave signal, GW190521, with a three-detector network signal-to-noise ratio of 14.7, and an estimated false-alarm rate of 1 in 4900~yr using a search sensitive to generic transients. If GW190521 is from a quasicircular binary inspiral, then the detected signal is consistent with the merger of two black holes with masses of 85_-14{\^}+21  M_⊙ and 66_-18{\^}+17  M_⊙ (90\% credible intervals). We infer that the primary black hole mass lies within the gap produced by (pulsational) pair-instability supernova processes, with only a 0.32\% probability of being below 65  M_⊙. We calculate the mass of the remnant to be 142_-16{\^}+28  M_⊙, which can be considered an intermediate mass black hole (IMBH). The luminosity distance of the source is 5.3_-2.6{\^}+2.4  Gpc, corresponding to a redshift of 0.82_-0.34{\^}+0.28. The inferred rate of mergers similar to GW190521 is 0.13_-0.11{\^}+0.30  Gpc{\^}-3 yr{\^}-1.