  Stellar heating causes atmospheres of close-in exoplanets to expand and
escape. These extended atmospheres are difficult to observe because their main
spectral signature - neutral hydrogen at ultraviolet wavelengths - is strongly
absorbed by interstellar medium. We report the detection of the near-infrared
triplet of neutral helium in the transiting warm Neptune-mass exoplanet
HAT-P-11b using ground-based, high-resolution observations. The helium feature
is repeatable over two independent transits, with an average absorption depth
of 1.08+/-0.05%. Interpreting absorption spectra with 3D simulations of the
planet's upper atmosphere suggests it extends beyond 5 planetary radii, with a
large scale height and a helium mass loss rate =< 3x10^5 g/s. A net blue-shift
of the absorption might be explained by high-altitude winds flowing at 3 km/s
from day to night-side.
