Broad absorption signatures from alkali metals, such as the sodium (Na I) and potassium (K I) resonance doublets, have long been predicted in the optical atmospheric spectra of cloud-free irradiated gas-giant exoplanets1,2,3. However, observations have only revealed the narrow cores of these features rather than the full pressure-broadened profiles4-6. Cloud and haze opacity at the day-night planetary terminator are considered responsible for obscuring the absorption-line wings, which hinders constraints on absolute atmospheric abundances7-9. Here we present an optical transmission spectrum for the hot-Saturn WASP-96b obtained with the Very Large Telescope, which exhibits the complete pressure-broadened profile of the sodium absorption feature. The spectrum is in excellent agreement with cloud-free, solar-abundance models assuming chemical equilibrium. We are able to measure a precise, absolute sodium abundance of logepsilon_Na=6.9+0.6-0.4, and use it as a proxy to the planets atmospheric metallicity relative to the solar value (Z_p/Z_star=2.3+8.9/--1.7). This result is consistent with the mass-metallicity trend observed for solar-system planets and exoplanets10-12.
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