  One of the targets of the recently launched Fermi Gamma-ray Space Telescope
is a diffuse gamma-ray background from dark-matter annihilation or decay in the
Galactic halo. N-body simulations and theoretical arguments suggest that the
dark matter in the Galactic halo may be clumped into substructure, rather than
smoothly distributed. Here we propose the gamma-ray-flux probability
distribution function (PDF) as a probe of substructure in the Galactic halo. We
calculate this PDF for a phenomenological model of halo substructure and
determine the regions of the substructure parameter space in which the PDF may
be distinguished from the PDF for a smooth distribution of dark matter. In
principle, the PDF allows a statistical detection of substructure, even if
individual halos cannot be detected. It may also allow detection of
substructure on the smallest microhalo mass scales, $\sim M_\oplus$, for
weakly-interacting massive particles (WIMPs). Furthermore, it may also provide
a method to measure the substructure mass function. However, an analysis that
assumes a typical halo substructure model and a conservative estimate of the
diffuse background suggests that the substructure PDF may not be detectable in
the lifespan of Fermi in the specific case that the WIMP is a neutralino.
Nevertheless, for a large range of substructure, WIMP annihilation, and diffuse
background models, PDF analysis may provide a clear signature of substructure.
