We report a facile method of synthesizing grain-boundary(GB)-rich platinum nanoparticle assembly. GBs are formed between platinum nanoparticles during their random collision and attachment in solution driven by water electrolysis. The GB-rich nanoparticle assembly exhibits ~400-fold higher catalytic hydrogen oxidation rate than platinum nanoparticles before assembly, enabling catalytic hydrogen sensing at room temperature without external heating. Our sensor also demonstrates fast response/recovery (~7 s at >1% H2), nearly no signal variation during a 280-hour-long stability test, and high selectivity toward hydrogen over 36 interference gases. Furthermore, this sensor can be easily fabricated from commercial thermometers at a low cost (< $5 per unit). Theoretical calculation results reveal that the high performance of GB-rich platinum nanoparticle assembly arises from tensile strain at the GBs.