The structure–property link is inherent to materials. Hexagonal Ge₄Se₃Te phase, with special Ge–Ge cationic bonding and distinctive from other IV–VI compounds, reveals thermoelectric (TE) performance far below the common values for IV–VI compounds. Here, it is shown that its TE performance can be substantially enhanced by phase modulation: increasing sintering temperature from 573 to 773 K leads to the decomposition of hexagonal phase into a composite of orthorhombic and rhombohedral phases, which significantly improves TE performance at T ≤ 550 K; Sb/Cd doping stabilizes the rhombohedral phase, compensates the hole concentration, and strengthens the phonon scattering, which further enhances the TE performance over the whole temperature range. The theoretical calculation reveals that the rhombohedral phase has a more favorable electronic band structure than the hexagonal phase for achieving higher electrical transport properties. The optimized TE performance is obtained in rhombohedral Ge_0.90Sb_0.08Cd_0.02Se_0.75Te_0.25, with a zT_max of 1.36 at 778 K and zT_ave of 0.73 from 322 to 778 K, which are among the highest values in the Se‐rich side of Ge(Se, Te) system, and one order higher than those in the hexagonal phase. The study sheds light on the validity of phase modulation for TE performance optimization in Ge(Se, Te)‐based material systems.