We studied photoluminescence of spatially indirect excitons (IXs) in a MoSe₂/WSe₂ van der Waals heterostructure. We observed a quasi-periodic triangular pattern of IXs with a characteristic spatial wavelength ∼2.6 μm. Our theoretical analysis using a Föppl-von Kármán theory finds an elastic instability that produces triangular patterns with micrometer-scale wavelengths. This mechanism is consistent with the wavelength, symmetry, excitation power dependence, and temperature dependence of the observed IX pattern.