  In this paper we generalize a well-known isomorphism between the space of
cusp forms of weight $k$ for a Fuchsian subgroup of the first kind $\Gamma
\subset\mathrm{SL}_{2}(\mathbb{R})$ and the space of certain Maa{\ss} forms of
weight $k$ for $\Gamma$ to an isomorphism between the space of Siegel cusp
forms of weight $k$ for a subgroup $\Gamma\subset\mathrm{Sp}_{n}(\mathbb{R})$,
which is commensurable with the Siegel modular group
$\mathrm{Sp}_{n}(\mathbb{Z})$, and a suitable space of Siegel-Maa{\ss} forms of
weight $k$ for $\Gamma$.
