  We report the Fermi Large Area Telescope (LAT) detection of a $\gamma$-ray
source at the position of SAX J1808.4$-$3658. This transient low-mass X-ray
binary contains an accreting millisecond puslar, which is only seen during its
month-long outbursts and likely switches to be rotation powered during its
quiescent state. Emission from the $\gamma$-ray source can be described by a
power law with an exponential cutoff, the characteristic form for pulsar
emission. Folding the source's 2.0--300 GeV photons at the binary orbital
period, a weak modulation is seen (with an H-test value of $\sim$17). In
addition, three sets of archival XMM-Newton data for the source field are
analyzed, and we find only one X-ray source with 3--4$\sigma$ flux variations
in the 2$\sigma$ error circle of the $\gamma$-ray source. However based on the
X-ray properties, this X-ray source is not likely a background AGN, the major
class of Fermi sources detected by LAT. These results support the possible
association between the $\gamma$-ray source and SAX J1808.4$-$3658 and thus the
scenario that the millisecond pulsar is rotation powered in the quiescent
state. Considering a source distance of 3.5 kpc for SAX J1808.4$-$3658, the
0.1--300 GeV luminosity is 5.7$\times 10^{33}$ erg s$^{-1}$, implying a
$\gamma$-ray conversion efficiency of 63\% for the pulsar in this binary.

High Energy Astrophysical Phenomena