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An unexpected phase, formed throughout the ferritic matrix of an Fe–Mn (1.62 wt% Mn) alloy during a nitriding treatment at 843 K for 8 h, is analysed. This new phase, labelled θ′, is a metastable Mn nitride. It adopts the form of plates, with length 200 nm and width 20 nm. Its crystal structure is established by electron microdiffraction in conjunction with group-theory analysis. This nitride crystallizes in the tetragonal system and belongs to the space group P 42/m 2/m 2/c with the following lattice parameters: 
= 2.876 Å and 
= 5.752 Å. The θ′-phase crystal lattice is oriented with respect to the surrounding ferritic matrix according to the cube-on-cube orientation relationship, namely: 
, 
and 
.
= 2.876 Å and 
= 5.752 Å. The θ′-phase crystal lattice is oriented with respect to the surrounding ferritic matrix according to the cube-on-cube orientation relationship, namely: 
, 
and 
.
