The LIGO detection of GW150914 provides an unprecedented opportunity to study the two-bodymotion of a compact-object binary in the large-velocity, highly nonlinear regime, and to witness the finalmerger of the binary and the excitation of uniquely relativistic modes of the gravitational field.We carry outseveral investigations to determine whether GW150914 is consistent with a binary black-hole merger ingeneral relativity. We find that the final remnant’s mass and spin, as determined from the low-frequency(inspiral) and high-frequency (postinspiral) phases of the signal, are mutually consistent with the binaryblack-hole solution in general relativity. Furthermore, the data following the peak of GW150914 areconsistent with the least-damped quasinormal mode inferred from the mass and spin of the remnant blackhole. By using waveform models that allow for parametrized general-relativity violations during theinspiral and merger phases, we perform quantitative tests on the gravitational-wave phase in the dynamicalregime and we determine the first empirical bounds on several high-order post-Newtonian coefficients.Weconstrain the graviton Compton wavelength, assuming that gravitons are dispersed in vacuum in the sameway as particles with mass, obtaining a 90%-confidence lower bound of 1013 km. In conclusion, within ourstatistical uncertainties, we find no evidence for violations of general relativity in the genuinely strong-fieldregime of gravity.