On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational-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 of accurate waveform models that describe gravitational waves from a compact binary system in general relativity. GW150914 was produced by a nearly equal mass binary black hole of \(36^{+5}_{-4} M_\odot\) and \(29^{+4}_{-4} M_\odot\); for each parameter we report the median value and the range of the 90% credible interval. The dimensionless 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 \(0.09^{+0.03}_{-0.04}\) assuming standard cosmology. The source location is constrained to an annulus section of \(610\) deg\(^2\), primarily in the southern hemisphere. The binary merges into a black hole of \(62^{+4}_{-4} M_\odot\) and spin \(0.67^{+0.05}_{-0.07}\). This black hole is significantly more massive than any other inferred from electromagnetic observations in the stellar-mass regime.