In this contribution, new data on the reversible Li⁺ intercalation in iron substituted sodium manganates are provided. Novel Na_2/3Mn_1−yFe_yO₂ (y = 0, 1/3 and 2/3) compounds with a P′3-type structure are prepared from freeze-dried citrate precursors at 500 °C. A new structural element is the development of three-layer oxygen stacking contrary to the well-known P2-type Na_2/3MnO₂ with a two-layer sequence. The effect of Fe additives on the structure of Na_2/3MnO₂ was examined by XRD powder diffraction and TEM analysis. The oxidation state and the distribution of transition metal ions in Na_2/3Mn_1−yFe_yO₂ were analysed using electron paramagnetic resonance spectroscopy. The lithium intercalation in Na_2/3Mn_1−yFe_yO₂ was investigated in two-electrode lithium cells of the type Li|LiPF₆ (EC:DMC)|Na_2/3Mn_1−yFe_yO₂. The stability of the layered phases during lithium intercalation was studied by ex situ Raman spectroscopy. It was found that the intermediate Na_2/3Mn_2/3Fe_1/3O₂ composition is able to intercalate Li⁺ reversibly in high amounts. Details of the structure and its stability during the Li⁺ intercalation are discussed.