  We report structural and electronic properties of Na$_2$Ni$_3$S$_4$, a
quasi-two-dimensional compound composed of alternating layers of
[Ni$_3$S$_4$]$^{2-}$ and Na$^{+}$. The compound features a remarkable Ni-based
kagome lattice with a square planar configuration of four surrounding S atoms
for each Ni atom. Magnetization and electrical measurements reveal a weak
paramagnetic insulator with a gap of about 0.5 eV. Our band structure
calculation highlights a set of topological flat bands of the kagome lattice
derived from the rotated d$_{xz}$-orbital with $C_\mathrm{3}$ + $T$ symmetry in
the presence of crystal-field splitting.
