The tetragonal magnetostriction constant, (3/2)λ₁₀₀, of Fe–Si single crystals was measured and was found to be structure dependent. Similar to that of Fe–Ge single crystals, (3/2)λ₁₀₀ is positive in the single phase A2 regime, becomes negative in the single phase D0₃ regime, and changes from positive to negative between the two regimes. Short-range order in the A2 regime decreases the magnetostriction prior to the onset of long range order. In the single phase regions of both A2 and D0₃, thermal history does not show any obvious effect on the magnetostriction, contrary to that found for Fe–Ga alloys. However, in the regions of phase mixture involving A2, B2, and D0₃ phases, quenching pushes the change in magnetostriction from positive to negative to higher Si contents.