Quantum batteries harness the unique properties of quantum mechanics to enhance energy storage compared to conventional batteries. In particular, they are predicted to undergo superextensive charging, where batteries with larger capacity actually take less time to charge. Up until now however, they have not been experimentally demonstrated, due to the challenges in quantum coherent control. Here we implement an array of two-level systems coupled to a photonic mode to realise a Dicke quantum battery. Our quantum battery is constructed with a microcavity formed by two dielectric mirrors enclosing a thin ﬁlm of a fluorescent molecular dye in a polymer matrix. We use ultrafast optical spectroscopy to time resolve the charging dynamics of the quantum battery at femtosecond resolution. We experimentally demonstrate superextensive increases in both charging power and storage capacity, in agreement with our theoretical modelling. We find that decoherence plays an important role in stabilising energy storage, analogous to the role that dissipation plays in photosynthesis. This experimental proof-of-concept is a major milestone towards the practical application of quantum batteries in quantum and conventional devices. Our work opens new opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies, including the enhancement of solar cell efficiencies.