  We report the detection of gamma-ray emission by the Fermi Large Area
Telescope (Fermi-LAT) towards the young massive star cluster RCW 38 in the
1-500 GeV photon energy range. We found spatially extended GeV emission towards
the direction of RCW 38, which is best modelled by a Gaussian disc of
0.23$\deg$ radius with a significance of the extension is $\sim 11.4 \sigma$.
Furthermore, the spatial correlation with the ionized and molecular gas content
favors the hadronic origin of the gamma-ray emission. The gamma-ray spectrum of
RCW 38 has a relatively hard photon index of $2.44 \pm 0.03$, which is similar
to other young massive star clusters. We argue that the diffuse GeV gamma-ray
emission in this region likely originates from the interaction of accelerated
protons in the stellar cluster with the ambient gas.
