  For a complex manifold $(M,J)$, an SKT (or pluriclosed) metric is a
$J$-Hermitian metric $g$ whose fundamental form $\omega:=g(J\cdot,\cdot)$
satisfies the condition $\partial\overline{\partial}\omega=0$. As such, an SKT
metric can be regarded as a natural generalization of a K\"{a}hler metric. In
this paper, the exceptional Lie group $G_2$ is equipped with a left-invariant
integrable almost complex structure $\mathcal{J}$ via the Samelson construction
and a 7-parameter family of $\mathcal{J}$-Hermitian metrics is constructed.
From this 7-parameter family, the members which are SKT are calculated. The
result is a 3-parameter family of left-invariant SKT metrics on $G_2$. As a
special case, the aforementioned family of SKT metrics contains all
bi-invariant metrics on $G_2$. In addition, this 3-parameter family of
left-invariant SKT metrics are also invariant under the right action of a
certain maximal torus $T$ of $G_2$. Conversely, it is shown that if $g$ is a
left-invariant $\mathcal{J}$-Hermitian metric on $G_2$ such that $g$ is
invariant under the right action of $T$ and for which $(g,\mathcal{J})$ is SKT,
then $g$ must belong to this 3-parameter family of left-invariant SKT metrics.
