  The QCD axion is a well-motivated addition to the standard model to solve the
strong $C\!P$ problem. If the axion acquires mass dominantly from a hidden
sector, it can be as heavy as $O(1)$ GeV, and the decay constant can be as low
as $O(100)$ GeV without running into the axion quality problem. We propose new
search strategies for such heavy QCD axions at the Belle II experiment, where
the axions are expected to be produced via $B\to K a$. We find that a
subsequent decay $a\to 3\pi$ with a displaced vertex leads to a unique signal
with essentially no background, and that a dedicated search can explore the
range $O(1$-$10)$ TeV of decay-constant values. We also show that $a\to
\gamma\gamma$ can cover a significant portion of currently unexplored region of
$150 \lesssim m_a \lesssim 500$ MeV.
