  We study the cosmological collider signatures in the Higgs-$R^2$ inflation
model. We consider two distinct types of signals: one originating from the
inflaton coupling to Standard Model fermions and gauge bosons, and another
arising from the isocurvature mode interaction with the inflaton. In the former
case, we determine that the signal magnitude is likely too small for detection
by upcoming probes, primarily due to suppression by both the Planck scale and
slow-roll parameters. However, we provide a detailed computation of the signal
which could be potentially applicable to various Higgs inflation variants. For
the isocurvature mode signals, we observe that the associated couplings remain
unsuppressed when the isocurvature mode is relatively light or comparable to
the inflationary scale. In this case, we study the Higgs-$R^2$ inflation
parameter space that corresponds to the quasi-single-field inflation regime and
find that the signal strength could be as large as $|f_{\rm NL}| > 1$, making
Higgs-$R^2$ inflation a viable candidate for observation by future 21-cm
surveys.
