  We study the connection between collider and dark matter phenomenology in the
singlet extension of the Georgi-Machacek model. In this framework, the singlet
scalar serves as a suitable thermal dark matter (DM) candidate. Our focus lies
on the region $v_{\chi}<1$ GeV, where $v_{\chi}$ is the common vacuum
expectation value of the neutral components of the scalar triplets of the
model. Setting bounds on the model parameters from theoretical, electroweak
precision and LHC experimental constraints, we find that the BSM Higgs sector
is highly constrained. Allowed values for the masses of the custodial
fiveplets, triplets and singlet are restricted to the range $140~ {\rm GeV }<
M_{H_5} < 350~ {\rm GeV }$, $150~ {\rm GeV }< M_{H_3} < 270 ~{\rm GeV }$ and
$145~ {\rm GeV }< M_{H} < 300~ {\rm GeV }$. The extended scalar sector provides
new channels for DM annihilation into BSM scalars that allow to satisfy the
observed relic density constraint while being consistent with direct DM
detection limits. The allowed region of the parameter space of the model can be
explored in the upcoming DM detection experiments, both direct and indirect. In
particular, the possible high values of BR$(H^0_5\to\gamma\gamma)$ can lead to
an indirect DM signal within the reach of CTA. The same feature also provides
the possibility of exploring the model at the High-Luminosity run of the LHC.
In a simple cut-based analysis, we find that a signal of about $4\sigma$
significance can be achieved in final states with at least two photons for one
of our benchmark points.
