  Dark matter particles, if sufficiently light, may be produced in decays of
the Higgs boson. This Letter presents a statistical combination of searches for
$H\to\textrm{invisible}$ decays where $H$ is produced according to the Standard
Model via vector boson fusion, $Z(\ell\ell)H$, and $W\!/\!Z(\textrm{had})H$,
all performed with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions
at a center-of-mass energy of $\sqrt s = 13$ TeV at the LHC. In combination
with the results at $\sqrt s = 7$ and 8 TeV, an exclusion limit on the
$H\to\textrm{invisible}$ branching ratio of $0.26 (0.17^{+0.07}_{-0.05})$ at
95% confidence level is observed (expected).
