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The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals^,,,,, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality^,,, and emergent phenomena, as seen in perovskite heterostructures^,,. However, separation of these layers from the growth substrate has proved challenging, limiting the manipulation capabilities of these heterostructures with respect to exfoliated materials. Here we present a general method to create freestanding perovskite membranes. The key is the epitaxial growth of water-soluble Sr₃Al₂O₆ on perovskite substrates, followed by in situ growth of films and heterostructures. Millimetre-size single-crystalline membranes are produced by etching the Sr₃Al₂O₆ layer in …