Intense research efforts have focused on two-dimensional (2D) materials in the last decade. Layered organic-inorganic hybrid perovskites, which were discovered more than 20 years ago and have proved to be promising semiconductor materials, have not been made into atomically thin sheets and investigated so far. In this talk, we report the solution-phase growth of single and few unit-cell-thick single crystalline 2D hybrid perovskites of (C₄H₉NH₃)₂PbBr₄ with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift compared to the bulk crystal. The high quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials. This study opens up the possibility of fundamental research into the synthesis and properties of atomically thin 2D hybrid perovskites, and introduces a new family of 2D semiconductors for nanoscale optoelectronic devices.