  Monolayer graphene embedded with transition metal nitride (i.e., $X$N$_4$)
has been experimentally synthesized recently, where a transition metal atom
together with four nitrogen atoms as a unit are embedded in graphene to form a
stable planar single-atom-thick structure. We provide a systematic study on the
structural, electronic and topological properties of these $X$N$_4$-embedded
graphene by utilizing both first-principles calculations and tight-binding
model. We find that $X$N$_4$-embedded graphene ($X$=Pt, Ir, Rh, Os) can open
topologically nontrivial band gaps that host \emph{two-dimensional}
$\mathbb{Z}_2$ topological insulators. We further show that the low-energy
bands near the band gaps can be perfectly captured by a modified Kane-Mele
model Hamiltonian. Our work not only provides concrete two-dimensional
materials that are very rare to realize \emph{two-dimensional} $\mathbb{Z}_2$
topological insulators, but also makes the graphene system to be realistic in
hosting Kane-Mele type $\mathbb{Z}_2$ topological insulators.
