Defect engineering is a versatile approach to modulate band and electronic structures as well as materials performance. Herein, metal–organic frameworks (MOFs) featuring controlled structural defects, namely UiO‐66‐NH2‐X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis), were synthesized to systematically investigate the effect of structural defects on photocatalytic properties. Remarkably, structural defects in MOFs are able to switch on the photocatalysis. The photocatalytic H2 production rate presents a volcano‐type trend with increasing structural defects, where Pt@UiO‐66‐NH2‐100 exhibits the highest activity. Ultrafast transient absorption spectroscopy unveils that UiO‐66‐NH2‐100 with moderate structural defects possesses the fastest relaxation kinetics and the highest charge separation efficiency, while excessive defects retard the relaxation and reduce charge separation efficiency. Volcano‐type trend: A series of metal–organic frameworks (MOFs) decorated with Pt nanoparticles, Pt@UiO‐66‐NH2‐X, were fabricated with increasing levels of structural defects in the MOF to investigate how defect levels affect photocatalysis. The catalysts exhibit an impressive volcano‐type trend in H2 production, maximizing at a moderate defect level.