Magnetic properties of ternary Cu(0–12.5 at. %) — Mn(55–42.5 at. %) — Al(45%) alloys have been studied in relation to the change of the tetragonal structure from the binary Mn&sngbnd;Al alloy to the ternary alloys. The substitution of Mn by Cu stabilizes the ferromagnetic tetragonal phase of the Mn&sngbnd;Al alloys. An increase of the Cu content brings about a decrease of the Curie temperature, the intrinsic coercive force, and the ratio of remanence to saturation induction. Correspondingly, the introduction of Cu into Mn&sngbnd;Al alloys first reduces the axial ratioc/aof the tetragonal phase from 1.30 in the 55 at. % Mn&sngbnd;Al alloy to 1.20 at 10 at. % Cu, and then converts the tetragonal structure into body‐centered cubic at more than 10 at. % Cu. In the latter case, the ternary alloys are ferromagnetic disregarding aging conditions.