  The origin of black hole mergers discovered by the LIGO and Virgo
gravitational-wave observatories is currently unknown. GW190521 is the heaviest
black hole merger detected so far. Its observed high mass and possible
spin-induced orbital precession could arise from the binary having formed
following a close encounter. An observational signature of close encounters is
eccentric binary orbit; however, this feature is currently difficult to
identify due to the lack of suitable gravitational waveforms. No eccentric
merger has been previously found. Here we report 611 numerical relativity
simulations covering the full eccentricity range and an estimation approach to
probe the eccentricity of mergers. Our set of simulations corresponds to $\sim
10^5$ waveforms, comparable to the number used in gravitational wave searches,
albeit with coarser mass-ratio and spin resolution. We applied our approach to
GW190521 and found that it is the most consistent with a highly eccentric
($e=0.69^{+0.17}_{-0.22}$; 90% credible level) merger within our set of
waveforms. This interpretation is supported over a non-eccentric merger with
$>10$ Odds ratio if $\gtrsim10\%$ of GW190521-like mergers are highly
eccentric. Detectable orbital eccentricity would be evidence against an
isolated binary origin, which is otherwise difficult to rule out based on
observed mass and spin.
