  We report the discovery of gamma-ray pulsations (> 0.1 GeV) from the young
radio and X-ray pulsar PSR J0205+6449 located in the Galactic supernova remnant
3C 58. Data in the gamma-ray band were acquired by the Large Area Telescope
aboard the Fermi Gamma-ray Space Telescope (formerly GLAST), while the radio
rotational ephemeris used to fold gamma-rays was obtained using both the Green
Bank Telescope and the Lovell telescope at Jodrell Bank. The light curve
consists of two peaks separated by 0.49 +/- 0.01 +/- 0.01 cycles which are
aligned with the X-ray peaks. The first gamma-ray peak trails the radio pulse
by 0.08 +/- 0.01 +/- 0.01, while its amplitude decreases with increasing energy
as for the other gamma-ray pulsars. Spectral analysis of the pulsed gamma-ray
emission suggests a simple power law of index -2.1 +/- 0.1 +/- 0.2 with an
exponential cut-off at 3.0 +1.1 -0.7 +/- 0.4 GeV. The first uncertainty is
statistical and the second is systematic. The integral gamma-ray photon flux
above 0.1 GeV is (13.7 +/- 1.4 +/- 3.0) x 10^(-8) /cm2/s, which implies for a
distance of 3.2 kpc and assuming a broad fan-like beam a luminosity of 8.3 x
10^(34) ergs/s and an efficiency eta of 0.3%. Finally, we report a 95% upper
limit on the flux of 1.7 x 10^(-8) /cm2/s for off-pulse emission from the
object.
