The microstructural changes and interdiffusion coefficients at the Sm2Fe17–Zn interface were investigated in this study. Sm2Fe17–Zn diffusion couples were annealed for 10 min at 320, 350, and 400°C, which are below the melting temperature of Zn (419°C), resulting in the interdiffusion of Zn, Fe, and Sm. At the interface annealed at 400°C, three diffusion regions were identified between the Sm2Fe17 and Zn phases. The thickest region was a Zn-rich region composed of polycrystalline δ- and ζ-Zn–Fe binary alloy phases and a ThMn12-type Sm(Zn,Fe)12 ternary alloy phase. The annealing time dependence of the thickness of the Zn-rich region at 400°C was measured, and the interdiffusion coefficient was evaluated as 7.3 × 10−13 m2s−1 using the Einstein–Smoluchowski equation. At the surface of the Sm2Fe17 phase, a region with a fiber-like microstructure was observed that consisted of two phases, α-(Zn,Fe) and Sm(Zn,Fe)12. Interestingly, these two phases exhibited a specific crystal orientational relationship of α-FeZn(10-1)//Sm(Zn,Fe)12(01-1). Between these two diffusion regions, a third region composed of nanosized polycrystalline grains of Γ-FeZn, α-FeZn, and Sm(Zn,Fe)12 was observed.The temperature dependence of the microstructural changes at the Sm2Fe17–Zn interface indicated that the microstructural changes proceed as follows. In the initial stage of diffusion, interdiffusion of Zn, Fe, and Sm lead to the formation of the Zn-rich region. As the interdiffusion progresses, Zn diffuses into the Sm2Fe17 phase, which decomposes to Sm(Zn,Fe)12 and α-FeZn phases with a fiber-like microstructure. Upon further Zn diffusion into the fiber-like region, Zn reacts with the α-FeZn phase to generate a polycrystalline region composed of Γ-FeZn, α-FeZn, and Sm(Zn,Fe)12 phases. To the best our knowledge, this is the first paper to report on the interdiffusion and detailed microstructural changes at the Sm2Fe17–Zn interface.