  Recently Belle II reported the first measurement of $B^+\to K^++{\rm
invisible (inv)}$, which is $2.7\sigma$ above the standard model (SM)
prediction. If confirmed, this calls for new physics beyond SM. In the SM, the
invisible particles are neutrino-anti-neutrino pairs. There are more
possibilities when going beyond the SM. In this work, we focus on decays to
dark matter (DM) and show that the $B\to K +\mathrm{inv}$ excess from Belle II
and DM relic density can be simultaneously explained in a simple extension of
the SM. The model introduces a real scalar singlet $\phi$ acting as a DM
candidate, and two heavy vector-like quarks $Q,D$ with the same quantum numbers
as the SM left-handed quark doublet and right-handed down-type quark singlet,
respectively. All these new particles are odd under a $\mathbb{Z}_2$ symmetry
while the SM particles are even. The model can successfully explain the Belle
II anomaly and DM relic density for TeV-scale heavy quarks with hierarchical
Yukawa couplings involving $b$ and $s$ quarks. At the same time, it can easily
satisfy other flavour physics constraints. Direct detection searches utilizing
the Migdal effect constrain some of the parameter space.
