  Discovered after the end of the Compton Gamma-Ray Observatory mission, the
radio pulsar PSR J2021+3651 was long considered a likely counterpart of the
high-energy gamma-ray source 2CG 075+00 = 3EG J2021+3716 = GeV J2020+3658, but
it could not be confirmed due to the lack of a contemporaneous radio pulsar
ephemeris to fold the sparse, archival gamma-ray photons. Here, we report the
discovery of gamma-ray pulsations from PSR J2021+3651 in the 100-1500 MeV range
using data from the AGILE satellite gathered over 8 months, folded on a densely
sampled, contemporaneous radio ephemeris obtained for this purpose at the Green
Bank Telescope. The gamma-ray pulse consists of two sharp peaks separated by
0.47+/-0.01 cycles. The single radio pulse leads the first gamma-ray peak by
0.165+/-0.010 cycles. These properties are similar to those of other gamma-ray
pulsars, and the phase relationship of the peaks can be interpreted in the
context of the outer-gap accelerator model for gamma-ray emission. Pulse-phase
resolved images show that there is only one dominant source, AGL J2020.5+3653 =
PSR J2021+3651 in the region previously containing confused sources 3EG
J2021+3716 and 3EG J2016+3657.
