A phenomenological model of high-temperature ferromagnetism in silicon-manganese alloys has been proposed taking into account phase separation in these alloys, where manganese-rich particles of the secondary phase (precipitate MnSi2 − z with z ≈ 0.25–0.30) are formed inside a manganese-depleted matrix of almost pure silicon. Precipitate MnSi2 − z is considered as the silicide MnSi1.7 containing a certain number of magnetic defects whose origin is due to the presence of weakly hybridized 3d orbitals of manganese. The silicide MnSi1.7 is a weak band ferromagnet in which strong fluctuations of the spin density (paramagnons) are present at a temperature much higher than its Curie temperature. It has been shown that the ferromagnetic exchange interactions between the magnetic moments of defects in precipitate exists due to thermal excitations of the spin density and the ferromagnetic order can appear at a temperature much higher than the Curie temperature of the silicide. The spatial structures and characteristics of this order have been described in the framework of the proposed approach for both homogeneous bulk precipitate and precipitate particles of various shapes and sizes. The short-range magnetic order near the bulk phase transition has been analyzed taking into account inhomogeneities in the distribution of magnetic defects in precipitate. The experimental data on the magnetic properties of silicon-manganese alloys have been interpreted in terms of the theoretical results obtained in this work.