Uncovering new structure-activity relationships for metal nanoparticle (NP) electrocatalysts is crucial for advancing many energy conversion technologies. Grain boundaries (GBs) could be used to stabilize unique active surfaces, but a quantitative correlation between GBs and catalytic activity has not been established. Here we use vapor deposition to prepare Au NPs on carbon nanotubes (Au/CNT). As deposited, the Au NPs have a relatively high density of GBs that are readily imaged by transmission electron microscopy (TEM); thermal annealing lowers the density in a controlled manner. We show that the surface-area-normalized activity for CO2 reduction is linearly correlated with GB surface density on Au/CNT, demonstrating that GB engineering is a powerful approach to improving the catalytic activity of metal NPs.