  The subspace sum graph $\mathcal{G}(\mathbb{V})$ on a finite dimensional
vector space $\mathbb{V}$ was introduced by Das [Subspace Sum Graph of a Vector
Space, arXiv:1702.08245], recently. The vertex set of $\mathcal{G}(\mathbb{V})$
consists of all the nontrivial proper subspaces of $\mathbb{V}$ and two
distinct vertices $W_1$ and $W_2$ are adjacent if and only if
$W_1+W_2=\mathbb{V}$. In that paper, some structural indices (e.g., diameter,
girth, connectivity, domination number, clique number and chromatic number)
were studied, but the characterization of automorphisms of
$\mathcal{G}(\mathbb{V})$ was left as one of further research topics. Motivated
by this, we in this paper characterize the automorphisms of
$\mathcal{G}(\mathbb{V})$ completely.
