  We report the superconductivity of a new quaternary compound ThMo$_2$Si$_2$C,
synthesized with the arc-melting technique. The compound crystallizes in a
tetragonal CeCr$_2$Si$_2$C-type structure with cell parameters of $a$ = 4.2296
\r{A}and $c$ = 5.3571 \r{A}. An interlayer Si-Si covalent bonding is suggested
by the atomic distance. The electrical resistivity and magnetic susceptibility
measurements indicate a Pauli-paramagnetic metal with dominant
electron-electron scattering in the normal-state. Bulk superconductivity at 2.2
K is demonstrated with a dimensionless specific-heat jump of
${\Delta}C/\gamma_{\rm n}T$ = 0.98. The superconducting parameters of the
critical magnetic fields, coherence length, penetration depth, and
superconducting energy gap are given.
