  Low energy antideuteron detection presents a unique channel for indirect
detection, targeting dark matter that annihilates into hadrons in a relatively
background-free way. Since the idea was first proposed, many WIMP-type models
have already been disfavored by direct detection experiments, and current
constraints indicate that any thermal relic candidates likely annihilate
through some hidden sector process. In this paper, we show that cosmic ray
antideuteron detection experiments represent one of the best ways to search for
hidden sector thermal relic dark matter, and in particular investigate a vector
portal dark matter that annihilates via a massive dark photon. We find that the
parameter space with thermal relic annihilation and $m_\chi > m_{A'} \gtrsim 20
\, \mathrm{GeV}$ is largely unconstrained, and near future antideuteron
experiment \GAPS will be able to probe models in this space with $m_\chi
\approx m_{A'}$ up to masses of $O(100\,\mathrm{GeV})$. Specifically the dark
matter models favored by the \textit{Fermi} Galactic center excess is expected
to be detected or constrained at the $5(3)-\sigma$ level assuming a optimistic
(conservative) propagation model.
