The GW190814 event was detected by the LIGO and Virgo gravitational wave detectors on 2019 August 14 at 21:10:39 UTC. It represents the coalescence of a 23 solar mass black hole with a compact object of 2.6 solar masses. The signal, observed in a three-detector network, has a signal-to-noise ratio of 25. The source was localized to 18.5 square degrees at a distance of 241 ± 41 Mpc. The event has the most unequal mass ratio measured with gravitational waves, 0.112 ± 0.008, and the secondary component is either the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system. The dimensionless spin of the primary black hole is tightly constrained to ≤ 0.07. General relativity tests reveal no measurable deviations from the theory, and higher-multipole emission is confirmed. The merger rate density for this new class of binary coalescence sources is estimated to be 1–23 Gpc⁻³ yr⁻¹. Astrophysical models suggest that binaries with similar mass ratios can form through several channels, but are unlikely to have formed in globular clusters. The combination of mass ratio, component masses, and merger rate challenges current models of compact-object binary formation and mass distribution. The event was identified as a loud two-detector event by the low-latency GSTLAL matched-filtering search pipeline. Subsequent analyses with full three-detector data confirmed the signal. No electromagnetic counterpart has been confirmed, consistent with the source's asymmetric masses and distance. Multimessenger follow-up observations across the electromagnetic spectrum and neutrino observations found no counterpart candidates. The significance of GW190814 was estimated using improved calibration and data-quality information, with false-alarm rates of 1 in 1.3 × 10³ yr and 1 in 8.1 yr for GSTLAL and PYCBC, respectively. The event was identified as a confident detection in analyses of data collected from August 7 to August 15, 2019. The source is classified as a neutron star-black hole (NSBH) system, with the secondary component having a mass below 3 solar masses. The event provides new insights into the formation and evolution of compact-object binaries and challenges current astrophysical models.The GW190814 event was detected by the LIGO and Virgo gravitational wave detectors on 2019 August 14 at 21:10:39 UTC. It represents the coalescence of a 23 solar mass black hole with a compact object of 2.6 solar masses. The signal, observed in a three-detector network, has a signal-to-noise ratio of 25. The source was localized to 18.5 square degrees at a distance of 241 ± 41 Mpc. The event has the most unequal mass ratio measured with gravitational waves, 0.112 ± 0.008, and the secondary component is either the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system. The dimensionless spin of the primary black hole is tightly constrained to ≤ 0.07. General relativity tests reveal no measurable deviations from the theory, and higher-multipole emission is confirmed. The merger rate density for this new class of binary coalescence sources is estimated to be 1–23 Gpc⁻³ yr⁻¹. Astrophysical models suggest that binaries with similar mass ratios can form through several channels, but are unlikely to have formed in globular clusters. The combination of mass ratio, component masses, and merger rate challenges current models of compact-object binary formation and mass distribution. The event was identified as a loud two-detector event by the low-latency GSTLAL matched-filtering search pipeline. Subsequent analyses with full three-detector data confirmed the signal. No electromagnetic counterpart has been confirmed, consistent with the source's asymmetric masses and distance. Multimessenger follow-up observations across the electromagnetic spectrum and neutrino observations found no counterpart candidates. The significance of GW190814 was estimated using improved calibration and data-quality information, with false-alarm rates of 1 in 1.3 × 10³ yr and 1 in 8.1 yr for GSTLAL and PYCBC, respectively. The event was identified as a confident detection in analyses of data collected from August 7 to August 15, 2019. The source is classified as a neutron star-black hole (NSBH) system, with the secondary component having a mass below 3 solar masses. The event provides new insights into the formation and evolution of compact-object binaries and challenges current astrophysical models.