Abstract: Grain boundary engineering (GBE) enables microstructural optimization and mechanical enhancement through precise control of grain boundary character distribution (GBCD), offering considerable potential for industrial applications. Appropriate GBE combined with aging treatment in Inconel 625 alloy leads to a marked increase in the fraction of Σ3 GBs and promotes effective precipitation of secondary phases. The influence of grain boundary (GB) type and triple-junctions (TJs) configuration on δ-phase precipitation behavior has been systematically examined using multiscale characterization, and the associated mechanisms are elucidated. The results have indicated that secondary phase precipitation has rarely been observed on most Σ3c GBs. The δ-phase precipitation propensity across different GB types has followed the ascending order: Σ3c<Σ3i<Σ9<Σ27<random grain boundaries (RBs). Meanwhile, on Σ3i GBs, the δ-phase has been observed to maintain an orientation relationship with the γ matrix described as 100δ//110γ and (020)δ//(111)γ. Moreover, the phenomena of the δ phases growing along Σ3c GBs are also observed on some Σ3c GBs. At different TJs, the elemental diffusion rate governs secondary phase formation along GBs, with J3-type TJs strongly suppressing precipitation. In summary, GBCD significantly influences δ-phase precipitation, thereby critically affecting the mechanical properties of nickel-based superalloys. This investigation offers new perspectives on optimizing nickel‑based superalloys via GBE and provides a basis for future industrial implementation.