Spatial photonic crystals (SPCs) are unique structures for light–matter interactions because they achieve a large and spatially periodic dielectric contrast on wavelength scales^{1, 2, 3–4}. Their temporal analogues, photonic time crystals (PTCs), promise similar advances by periodically modulating optical properties in time^{5, 6, 7, 8, 9, 10–11}, but require strong, ultrafast modulation, which is challenging to obtain experimentally^{5,12, 13, 14–15}. Driven metamaterials have been considered as a route to realize PTCs, yet all-optical implementations have remained unknown because of the challenge of achieving modulation on such short timescales. Here we demonstrate the all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at terahertz frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the kinetic energy of the carriers and effective mass reaching up to 80% of their rest mass. Our spectroscopic measurements show a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% (refs. ^16,17), and we predict plasmonic lasing to be within experimental reach. These results establish a robust platform for time-domain photonics in plasmonic systems.