<p>Multiple exciton generation (MEG) from single-photon absorption can enhance quantum efficiencies in light emission and energy conversion. However, its practical application is limited by high photon-energy thresholds, large material bandgaps, and rapid exciton recombination. Here, we report the observation of interlayer exciton multiplication (IXM) in twisted van der Waals heterobilayers, where the process threshold is as low as twice the type-II bandgap and the resulting multiple interlayer excitons (IXs) exhibit nanosecond-scale lifetimes. These properties directly address the core challenges of MEG, leading to a significant boost in both interlayer exciton emission and photocurrent quantum efficiency. Theoretical calculations confirm the experimental results, revealing that low-threshold IXM is facilitated by interlayer hot-carrier scattering and is accompanied by attractive IX interactions, even at larger twist angles where momentum mismatch is significant. Twist-angle-induced momentum mismatches have a minimal impact on the IXM threshold due to strong hot-carrier interlayer transfer and efficient intervalley impact ionization. The IXM efficiency reaches ~90% in small-twist-angle heterobilayers but decreases at larger angles and higher pump photon energies, a trend attributed to reduced interlayer hot-carrier Coulomb scattering. Furthermore, we demonstrate photocurrent multiplication driven by IXM. Applied electric fields further lower the IXM threshold and increase the IX yield, enabling a twofold enhancement in quantum efficiency and a fourfold increase in responsivity in a self-powered heterobilayer photodiode. Our results establish IXM as a promising mechanism for high-efficiency carrier-multiplication optoelectronics and provide insights into the many-body physics of stable multiple excitons.</p>