Negative thermal expansion (NTE), where materials contract rather than expand with temperature rise, is crucial for applications requiring thermal stability, and its regulation holds significant technological promise. This work demonstrates that nanosizing Cu₂P₂O₇ through high-energy ball milling suppresses its NTE by 30.6%, with the average volumetric thermal expansion coefficient decreasing from −27.69 × 10⁻⁶ K⁻¹ (bulk, 5–375 K) to −19.20 × 10⁻⁶ K⁻¹ (nano, 87–440 K), while broadening the NTE temperature range by 65 K. Structural analyses reveal that reduced distortion of CuO₄ polyhedra in nanocrystals slows the α-to-β phase transition, mitigating NTE. The study highlights nanosizing as an effective strategy to tune NTE in phase-transition materials by alleviating internal stress, offering a novel approach for thermal expansion regulation.