Emerging examples of direct plasmonic sensors are plasmonic hydrogen sensors based on palladium (Pd) nanoparticles and their alloys. These devices feature spark-free and roomtemperature operation, efficient remote readout with small footprints, subsecond response time with excellent resistance to cross-contaminating and deactivating gases, and long-term stability. Mechanistically, these sensors rely on the barrierless dissociation of H 2 molecules at the surface of Pd nanoparticles and the subsequent intercalation of H atoms into the metal lattice. The corresponding change in dielectric function between pure Pd and Pd hydride leads to shifts in the localized surface plasmon resonance (LSPR) spectra of Pd nanoparticles, which are linearly proportional to the hydrogen concentration inside the particles. Unfortunately, due to the lossy nature of palladium, the LSPR of Pd nanoparticles is broad, with full-widths at half maximum (FWHMs) typically >300 nm for nanostructures with plasmonic spectra at visible or near infrared frequencies. Consequently, these broad peaks introduce inaccuracies in the determination of the sensing readout peak position, λ peak , leading to higher signal noise, σ, and thus higher limits of detection (LoD), defined as the lowest analyte concentration measurable with a signal larger than 3σ. In fact, the detection limit still remains a significant challenge for plasmonic (and optical) sensors, with the state-of-the-art only at single-digit ppm; a comparably inferior performance than electrical sensors where ppb detection limit has been reported (Supplementary Table ). While ppm hydrogen sensitivity is appropriate for some applications, an ultralow detection limit, coupled with the abovementioned advantages of plasmonic sensing, is crucial for various application requiring local and early detection, such as hydrogen embrittlement in engineering structural materials, and intragastric hydrogen production in bacterial infections. Here, we overcome the sensitivity bottleneck of optical hydrogen sensors by designing and experimentally demonstrating a sensor capable of detecting hydrogen gas down to the ppb level.