A giant, anomalous piezoresponse of fully depleted silicon-on-insulator devices under mechanical stress is demonstrated by impedance spectroscopy. This piezoresponse strongly depends on the measurement frequency, ω, and consists of both a piezoresistance (PZR) and a piezocapacitance, whose maximum values are −1100 × 10−11 and −900 × 10−11 Pa−1, respectively. These values should be compared with the usual bulk PZR in p-type silicon, 70 × 10−11 Pa−1. The observations are well described by models of space-charge-limited hole currents in the presence of fast electronic traps having stress-dependent capture rates (ωc) and emission rates. Under steady-state conditions (i.e., when ω ωc), where the impedancespectroscopy measurements yield results that are directly comparable with those of previously published reports of PZR in depleted, silicon nano-objects, the overall piezoresponse is just the usual, bulk silicon PZR. Anomalous PZR is observed only under non-steady-state conditions when ω ≈ ωc, with a symmetry suggesting that the electromechanically active fast traps are native Pb0 interface defects. The observations suggest new functionalities for fully depleted silicon-on-insulator, and shed light on the debate over the PZR of carrier-depleted nanosilicon.