Aims. The gamma-ray burst (GRB) 221009A, with its extreme brightness, has provided the opportunity to explore GRB prompt and afterglow emission behavior on short timescales with high statistics. In conjunction with detection up to very-high-energy gamma rays, studies of this event shed light on the emission processes at work in the initial phases of GRBs' emission. Methods. Using INTEGRAL/IBIS's soft gamma-ray detector, PICsIT (200−2600 keV), we studied the temporal and spectral evolution during the prompt phase and the early afterglow period. Results. We found a "flux-tracking" behavior with the source spectrum "softer" when brighter. However, the relationship between the spectral index and the flux changes during the burst. The PICsIT light curve shows that afterglow emission begins to dominate at ∼ T 0 + 630 s and decays with a slope of 1.6 ± 0.2, consistent with the slopes reported at soft X-rays.