This work demonstrates a facile and scalable approach for synthesizing a GB-rich Pt NP assembly and its practical use as an exceptionally high-performance catalytic hydrogen sensor that operates at room temperature. Furthermore, given the recently discovered high activities of GBs for other catalytic reactions such as CO2 electroreduction and methane activation, this synthetic approach will be powerful for studying the activity at GBs between NPs because the crystallite size of the NP precursors is largely retained in the NP-assembly, making the comparison between the activities with and without GBs straightforward.