Phase-locking of oscillators leads to super-radiant amplification of the emission power. This is particularly important for development of terahertz sources, which suffer from low emission efficiency. In this work we study large Josephson junction arrays containing several thousand Nb-based junctions. Using low-temperature scanning laser microscopy, we observe that at certain bias conditions two-dimensional standing-wave patterns are formed, manifesting themselves as global synchronization of the arrays. Analysis of standing waves indicates that they are formed by surface plasmon-type electromagnetic waves propagating at the electrode-substrate interface. Thus, we demonstrate that surface waves provide an effective mechanism for long-range coupling and phase-locking of large junction arrays.