  Dark matter particles with Planck-scale mass ($\simeq10^{19}\text{GeV}/c^2$)
arise in well-motivated theories and could be produced by several cosmological
mechanisms. Using a blind analysis of data collected over a 813 d live time
with DEAP-3600, a 3.3 t single-phase liquid argon-based dark matter experiment
at SNOLAB, a search for supermassive dark matter was performed, looking for
multiple-scatter signals. No candidate signal events were observed, leading to
the first direct detection constraints on Planck-scale mass dark matter.
Leading limits constrain dark matter masses between $8.3\times10^{6}$ and
$1.2\times10^{19} \text{GeV}/c^2$, and cross sections for scattering on
$^{40}$Ar between $1.0\times10^{-23}$ and $2.4\times10^{-18} \text{cm}^2$.
These are used to constrain two composite dark matter models.
