  We report the discovery and detailed investigation of superconductivity in
Mo$_4$Ga$_{20}$As. Mo$_4$Ga$_{20}$As crystallizes in the space group of $I4/m$
(No. 87), with lattice parameters $a$ = 12.86352 \AA and $c$ = 5.30031 \AA. The
resistivity, magnetization, and specific heat data reveal Mo$_4$Ga$_{20}$As to
be a type-II superconductor with $T_c$ = 5.6 K. The upper and lower critical
fields are estimated to be 2.78 T and 22.0 mT, respectively. In addition,
electron-phonon coupling in Mo$_4$Ga$_{20}$As is possibly stronger than the BCS
weak-coupling limit. First-principles calculations suggest the Fermi level
being dominated by the Mo-4$d$ and Ga-4$p$ orbitals.
