NPM1-mutated (NPM1-mut) acute myeloid leukemia (AML) is generally associated with a more favorable outcome, although the presence of additional gene mutations can influence patient prognosis. We analyzed intensively-treated adult NPM1-mut AML patients included in the HARMONY Alliance database. A newly developed risk classification, which included combinations of co-mutations in FLT3-ITD, DNMT3A, IDH1/IDH2, and TET2 genes, was applied to a training cohort of NPM1-mut AML patients included in clinical trials (n = 1001), an internal validation cohort more representative of real-world settings (n = 762), and an external validation cohort enrolled in UK-NCRI trials (n = 585). The HARMONY classification considered 51.8% of the NPM1-mut AML training cohort patients as favorable, 24.8% as intermediate, and 23.4% as adverse risk, with median overall survival (OS) of 14.4, 2.2, and 0.9 years, respectively; p < 0.001), thereby reclassifying 42.7% of NPM1-mut patients into a different European LeukemiaNet (ELN) 2022 risk category. These results were confirmed both in an internal and external validation cohort. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) in first complete remission (CR1) showed the highest benefit in the NPM1-mut adverse-risk subgroup. The HARMONY classification provides the basis for a refined genetic risk stratification for adult NPM1-mut AML with potential clinical impact on allo-HSCT decision-making.