  When two black holes merge, the late stage of gravitational wave emission is
a superposition of exponentially damped sinusoids. According to the black hole
no-hair theorem, this ringdown spectrum depends only on the mass and angular
momentum of the final black hole. An observation of more than one ringdown mode
can test this fundamental prediction of general relativity. Here we provide
strong observational evidence for a multimode black hole ringdown spectrum
using the gravitational wave event GW190521, with a maximum Bayes factor of
$56\pm1$ ($1\sigma$ uncertainty) preferring two fundamental modes over one. The
dominant mode is the $\ell=m=2$ harmonic, and the sub-dominant mode corresponds
to the $\ell=m=3$ harmonic. The amplitude of this mode relative to the dominant
harmonic is estimated to be $A_{330}/A_{220} = 0.2^{+0.2}_{-0.1}$. We estimate
the redshifted mass and dimensionless spin of the final black hole as
$330_{-40}^{+30}~\mathrm{M}_{\odot}$ and $0.86_{-0.11}^{+0.06}$, respectively.
We find that the final black hole is consistent with the no hair theorem and
constrain the fractional deviation from general relativity of the sub-dominant
mode's frequency to be $-0.01^{+0.08}_{-0.09}$.
