  Using the data from the Large Area Telescope on board the Fermi Gamma-ray
Space Telescope, we have searched for the gamma-ray pulsations from the
direction of globular cluster M28 (NGC 6626). We report the discovery of a
signal with the frequency consistent with that of the energetic millisecond
pulsar (MSP) PSR B1821-24 in M28. A weighted H-test test statisic (TS) of 28.8
is attained which corresponds to a chance probability of ~1e-5 (4.3-sigma
detection). With a phase-resolved analysis, the pulsed component is found to
contribute ~25% of the total observed gamma-ray emission from the cluster. On
the other hand, the unpulsed level provides a constraint for the underlying MSP
population and the fundamental plane relations for the scenario of inverse
Compton scattering. Follow-up timing observations in radio/X-ray are encouraged
for further investigating this periodic signal candidate.
