In summary, we have used an inverse nanophotonic design approach to identify and experimentally demonstrate an ultrasensitive plasmonic hydrogen detector based on collective resonances in periodic arrays of palladium nanoparticles. The optimized sensor displays a nontrivial balance between a large optical response upon hydrogenation and narrow spectral features. The measured ppb limit of detection is an order of magnitude lower than any previous optical hydrogen sensor and becomes competitive with the more mature electrical sensors (Supplementary Table ). The genericity of our strategy allows it to be combined with other optimization approaches, including the use of more sensitive transduction materials such as PdAu, (eightfold more sensitive than Pd at low H 2 concentrations) or PdTa alloys, and advanced data fittings capable of producing lower signal noise. Furthermore, we have so far only explored a simple and generic figure-of-merit parameter during our optimization, namely the peaks shift divided by the linewidth. Our inverse design approach, however, also permits the optimization of nanoparticle arrays for sensing platforms using different readouts such as singlewavelength mode devices, opening the door to low-cost, ultrasensitive platforms. Beyond hydrogen sensing, our approach can be extended to arrays of surface-functionalized nanoparticles with resonances that are sensitive to the adsorption of specific gasses via refractive index effects or chemical interface damping, with the potential to address a wider range of societal needs, from home safety to urban air pollution monitoring. 56