  A colored heavy particle with sufficiently small width may form
non-relativistic bound states when they are produced at the large hadron
collider\,(LHC), and they can annihilate into a diphoton final state. The
invariant mass of the diphoton would be around twice of the colored particle
mass. In this paper, we study if such bound state can be responsible for the
750 GeV diphoton excess reported by ATLAS and CMS. We found that the best-fit
signal cross section is obtained for the SU(2)$_L$ singlet colored fermion $X$
with $Y_X=4/3$. Having such an exotic hypercharge, the particle is expected to
decay through some higher dimensional operators, consistent with the small
width assumption. The decay of $X$ may involve a stable particle $\chi$, if
both $X$ and $\chi$ are odd under some conserved $Z_2$ symmetry. In that case,
the particle $X$ suffers from the constraints of jets + missing $E_T$ searches
by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still
survives if the mass difference between $X$ and $\chi$ is above $\sim$ 30 GeV
for $m_X \sim 375$ GeV. Even assuming pair annihilation of $\chi$ is small, the
relic density of $\chi$ is small enough if the mass difference between $X$ and
$\chi$ is smaller than $\sim$ 40 GeV.
