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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 ofafterglow emission at such early time at other wavelengths. Here we report thediscovery of a hidden afterglow component during the prompt emission phase withFermi Gamma-Ray Burst Monitor (GBM) observations. We analyze the spectralevolution of the X-ray/$\gamma$-ray emission of GRB 221009A measured by GBMduring 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 GBMtrigger time). We find that the spectra at the dips transit from the Bandfunction to a power-law function, indicating a transition from the promptemission to the afterglow. After $\sim T_{0}+ 660 \rm~s$, the spectrum is welldescribed by a power-law function and the afterglow becomes dominant.Remarkably, the underlying afterglow emission at the dips smoothly connect withthe afterglow after $\sim T_{0}+ 660 \rm~s$. The entire afterglow emissionmeasured 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 ofthis power-law decay indicates that the afterglow peak of GRB 221009A should beearlier than $T_{0}+300 \rm ~s$. We also test the possible presence of a jetbreak in the early afterglow light curve, finding that both the jet break modeland single power-law decay model are consistent with the GBM data. The twomodels can not be distinguished with the GBM data alone because the inferredjet break time is quite close to the end of GBM observations.

GRB 221009A: revealing a hidden afterglow during the prompt emission phase with Fermi-GBM observations