Significance: The ability to control light has driven advances from optical microscopy to fast communication. Metal nanoparticles can trap light at the nanoscale in the form of surface plasmons; these localized excitations are important in sensing and designing miniaturized photonic devices. Although very narrow plasmon linewidths are desirable, they tend to be broad because of limitations in fabrication methods. Here, we describe how to achieve ultranarrow plasmon resonances with linewidths that approach the calculated limit by thermally annealing 2D lattices of metal nanoparticles. Our results indicate that exquisite control over structural features of nanoparticles can dramatically improve their plasmonic response and open promising strategies for manipulating light–matter interactions at the nanoscale.