On August 17, 2017 at 12∶41:04 UTC the Advanced LIGO and Advanced Virgo gravitational-wavedetectors made their first observation of a binary neutron star inspiral. The signal, GW170817, was detectedwith a combined signal-to-noise ratio of 32.4 and a false-alarm-rate estimate of less than one per8.0 × 104 years. We infer the component masses of the binary to be between 0.86 and 2.26 M⊙, inagreement with masses of known neutron stars. Restricting the component spins to the range inferred inbinary neutron stars, we find the component masses to be in the range 1.17–1.60 M⊙, with the total mass ofthe system 2.74þ0.04−0.01 M⊙. The source was localized within a sky region of 28 deg2 (90% probability) andhad a luminosity distance of 40þ8−14 Mpc, the closest and most precisely localized gravitational-wave signalyet. The association with the γ-ray burst GRB 170817A, detected by Fermi-GBM 1.7 s after thecoalescence, corroborates the hypothesis of a neutron star merger and provides the first direct evidence of alink between these mergers and short γ-ray bursts. Subsequent identification of transient counterpartsacross the electromagnetic spectrum in the same location further supports the interpretation of this event asa neutron star merger. This unprecedented joint gravitational and electromagnetic observation providesinsight into astrophysics, dense matter, gravitation, and cosmology.