  Recently, LHAASO reported the detection of brightest-of-all-time GRB 221009A,
revealing the early onset of a TeV afterglow. However, there is no evidence of
afterglow emission at such early time at other wavelengths. Here we report the
discovery of a hidden afterglow component during the prompt emission phase with
Fermi Gamma-Ray Burst Monitor (GBM) observations. We analyze the spectral
evolution of the X-ray/$\gamma$-ray emission of GRB 221009A measured by GBM
during the dips of two prompt emission pulses (i.e., intervals
$T_{0}+[300-328]\rm~s$ and $T_{0}+[338-378]\rm~s$, where $T_0$ is the GBM
trigger time). We find that the spectra at the dips transit from the Band
function to a power-law function, indicating a transition from the prompt
emission to the afterglow. After $\sim T_{0}+ 660 \rm~s$, the spectrum is well
described by a power-law function and the afterglow becomes dominant.
Remarkably, the underlying afterglow emission at the dips smoothly connect with
the afterglow after $\sim T_{0}+ 660 \rm~s$. The entire afterglow emission
measured by GBM can be fitted by a power-law function $F\sim t^{-0.95\pm0.05}$,
where $t$ is the time since the first main pulse at $T^*=T_0+226~{\rm s}$,
consistent with the TeV afterglow decay measured by LHAASO. The start time of
this power-law decay indicates that the afterglow peak of GRB 221009A should be
earlier than $T_{0}+300 \rm ~s$. We also test the possible presence of a jet
break in the early afterglow light curve, finding that both the jet break model
and single power-law decay model are consistent with the GBM data. The two
models can not be distinguished with the GBM data alone because the inferred
jet break time is quite close to the end of GBM observations.
