Formation of episodic memory requires precise information routing in the hippocampus. For more than a hundred years, the trisynaptic circuit has been considered the main anatomical substrate of the hippocampal circuitry responsible for learning and memory. However, the hippocampus includes a region located between CA3 and CA1, named “CA2”, whose intrahippocampal connectivity has remained elusive. Here, by using cell type-specific transgenic mice, viral tracers, optogenetics and patch-clamp recordings we examined CA2 circuits in the hippocampus. Our findings redefine hippocampal functional anatomy, allowing a deeper understanding of the biology of learning and memory.