<p>Broad-area semiconductor lasers are used in many high-power applications; however, their spatio-temporal dynamics are complex and intrinsically unstable due to the interaction between transverse lasing modes. Here a dynamical and ultrahigh-resolution spatio-spectral analysis of commercial broad-area lasers reveals multiplets of phase-locked first- and second-order transverse modes that are spontaneously created by the nonlinear dynamics of the laser. Phase locking between modes of different transverse order is confirmed by comparing the linewidths of the lasing modes with that of their beat note and by a direct measurement of their phase fluctuation correlations. The spontaneous phase locking is unexpected since the overall dynamics are unstable and the system lacks any intentional feature to induce locking. This partially synchronized dynamical state with groups of coexisting synchronized and unsynchronized laser modes is similar to chimera states found in networks of coupled oscillators, indicating that such states may exist in a wider range of systems than previously assumed.</p>