  Electron density oscillations with acoustic dispersions and sustained at
boundaries between different media provide information about surface and
interface properties of hetero-structures. In ultra-thin metallic films these
plasmonic excitations are heavily damped. Superconductivity is predicted to
reduce dissipation allowing detection of these resonances. Emerging low-loss
interface Cooper-pair waves have been studied before, however, the observation
of surface-confined Josephson plasmons has remained elusive. Here, we report on
generation and coupling to these excitations in an ultrathin single-crystal
film of high-temperature superconductor La1.85Sr0.15CuO4. The film becomes
brighter than Au below the critical temperature when probed with sub-gap THz
photons. We show that the enhanced signal in the superconducting state, which
can be visualized with a spatial resolution better than {\lambda}/3,000,
originates from near-field coupling of light to surface Josephson plasmons. Our
results open a path towards non-invasive investigation of enhanced
superconductivity in artificial multilayers, buried interface states in
topological hetero-structures, and non-linear phenomena in Josephson devices.
