Plasmons in graphene(1-4) can be tuned by using electrostatic gating or chemical doping(5-7), and the ability to confine plasmons in very small regions could have applications in optoelectronics(8,9), plasmonics(10,11) and transformation optics(12). However, little is known about how atomic-scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to around 10 nm (refs 13-15). Here, we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss spectrum imaging in an aberration-corrected scanning transmission electron microscope, we find that a single point defect can act as an atomic antenna in the petahertz (10(15) Hz) frequency range, leading to surface plasmon resonances at the subnanometre scale.