0_5483 Two-dimensional moiré superlattices provide a highly tunable platform to study strongly correlated physics. In particular, the moiré superlattices of two-dimensional semiconductor heterojunctions have been shown to host tunable correlated electronic states such as a Mott insulator and generalized Wigner crystals. Here we report the observation of an excitonic insulator, a correlated state with strongly bound electrons and holes, in an angle-aligned monolayer WS₂/bilayer WSe₂ moiré superlattice. The moiré coupling induces a flat miniband on the valence-band side only in the first WSe₂ layer interfacing WS₂. The electrostatically introduced holes first fill this miniband and form a Mott insulator when the carrier density corresponds to one hole per moiré supercell. By applying a vertical electric field, we tune the valence band in the second WSe₂ layer to overlap with the moiré miniband in the first WSe₂ layer, realizing the coexistence of electrons and holes at equilibrium, which are bound as excitons due to a strong Coulomb interaction. We show that this new bound state is an excitonic insulator with a transition temperature as high as 90 K. Our study demonstrates a moiré system for the study of correlated many-body physics in two dimensions.