An ideal n 3 member of the group of Ruddlesden–Popper (RP) phases Ca₄Mn₃O₁₀ was obtained by solid-state reaction under high pressure. This phase has been studied by transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy, and parallel electron energy-loss spectroscopy. The lattice parameters are derived as a b 0.37 nm and c 2.69 nm with the space group I4/mmm, which is the space group of the ideal RP phase.