On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected agravitational-wave transient (GW150914); we characterize the properties of the source and its parameters.The data around the time of the event were analyzed coherently across the LIGO network using a suite ofaccurate waveform models that describe gravitational waves from a compact binary system in generalrelativity. GW150914 was produced by a nearly equal mass binary black hole of masses 36+5−4M⊙ and29+4−4M⊙; for each parameter we report the median value and the range of the 90% credible interval. Thedimensionless spin magnitude of the more massive black hole is bound to be < 0.7 (at 90% probability).The luminosity distance to the source is 410+160−180 Mpc, corresponding to a redshift 0.09+0.03−0.04 assumingstandard cosmology. The source location is constrained to an annulus section of 610 deg^2, primarilyin the southern hemisphere. The binary merges into a black hole of mass 62+4−4M⊙ and spin 0.67+0.05−0.07 .This black hole is significantly more massive than any other inferred from electromagnetic observations inthe stellar-mass regime.