Although high-temperature superconductors are a complex and diverse family of materials, they all adopt a layered crystal structure, in which two-dimensional lattices stack together to form the three-dimensional bulk. This curious fact begs the question: does high-temperature superconductivity (HTS) exist in an isolated monolayer, and if so, is the two-dimensional HTS—and various other correlated phenomena related to HTS—different from its three-dimensional counterpart? The answer to these questions may provide important insights on the role of dimensionality in HTS. Here, we fabricate atomically thin cuprate Bi₂Sr₂CaCu₂O_8+δ that retains HTS down to monolayer (i.e. half unit cell) limit. The electronic structure of monolayer Bi₂Sr₂CaCu₂O_8+δ is probed with scanning tunneling microscopy and spectroscopy. Survey of the electronic phases – superconductivity, pseudogap, charge order and Mott insulating state – reveals that they are indistinguishable from those in the bulk. Out results, therefore, indicates that essential physics of HTS is contained in a monolayer Bi₂Sr₂CaCu₂O_8+δ.