  We present observations of HESS J1640-465 with the Fermi-LAT. The source is
detected with high confidence as an emitter of high-energy gamma-rays. The
spectrum lacks any evidence for the characteristic cutoff associated with
emission from pulsars, indicating that the emission arises primarily from the
pulsar wind nebula. Broadband modeling implies an evolved nebula with a low
magnetic field resulting in a high gamma-ray to X-ray flux ratio. The Fermi
emission exceeds predictions of the broadband model, and has a steeper
spectrum, possibly resulting from a distinct excess of low energy electrons
similar to what is inferred for both the Vela X and Crab pulsar wind nebulae.
