The paper reports the observation of gravitational waves from the coalescence of two stellar-mass black holes, labeled GW151226, detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on December 26, 2015. The signal, lasting approximately 1 second, increased in frequency and amplitude over about 55 cycles from 35 to 450 Hz, with a peak gravitational strain of \(3.4 \pm 0.9 \times 10^{-32}\). The initial masses of the black holes were estimated to be 14.2+3.4-2 M⊙ and 7.5+2.4-2 M⊙, and the final black hole mass was 20.8+6.1-0.4 M⊙. At least one of the black holes had a spin magnitude greater than 0.2. The source was located at a luminosity distance of 440+180-190 Mpc, corresponding to a redshift of 0.09+0.04-0.04. The detection provides improved constraints on stellar populations and deviations from general relativity.The paper reports the observation of gravitational waves from the coalescence of two stellar-mass black holes, labeled GW151226, detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) on December 26, 2015. The signal, lasting approximately 1 second, increased in frequency and amplitude over about 55 cycles from 35 to 450 Hz, with a peak gravitational strain of \(3.4 \pm 0.9 \times 10^{-32}\). The initial masses of the black holes were estimated to be 14.2+3.4-2 M⊙ and 7.5+2.4-2 M⊙, and the final black hole mass was 20.8+6.1-0.4 M⊙. At least one of the black holes had a spin magnitude greater than 0.2. The source was located at a luminosity distance of 440+180-190 Mpc, corresponding to a redshift of 0.09+0.04-0.04. The detection provides improved constraints on stellar populations and deviations from general relativity.