Tumor heterogeneity was evaluated by computing stable CT tumor habitats with unsupervised learning on repeatable and reproducible three-dimensional radiomics features in lung and liver cancer lesions. Purpose: To identify precise three-dimensional radiomics features in CT images that enable computation of stable and biologically meaningful habitats with machine learning for cancer heterogeneity assessment. Materials and Methods: This retrospective study included 2436 liver or lung lesions from 605 CT scans (November 2010–December 2021) in 331 patients with cancer (mean age, 64.5 years ± 10.1 [SD]; 185 male patients). Three-dimensional radiomics were computed from original and perturbed (simulated retest) images with different combinations of feature computation kernel radius and bin size. The lower 95% confidence limit (LCL) of the intraclass correlation coefficient (ICC) was used to measure repeatability and reproducibility. Precise features were identified by combining repeatability and reproducibility results (LCL of ICC ≥ 0.50). Habitats were obtained with Gaussian mixture models in original and perturbed data using precise radiomics features and compared with habitats obtained using all features. The Dice similarity coefficient (DSC) was used to assess habitat stability. Biologic correlates of CT habitats were explored in a case study, with a cohort of 13 patients with CT, multiparametric MRI, and tumor biopsies. Results: Three-dimensional radiomics showed poor repeatability (LCL of ICC: median [IQR], 0.442 [0.312–0.516]) and poor reproducibility against kernel radius (LCL of ICC: median [IQR], 0.440 [0.33–0.526]) but excellent reproducibility against bin size (LCL of ICC: median [IQR], 0.929 [0.853–0.988]). Twenty-six radiomics features were precise, differing in lung and liver lesions. Habitats obtained with precise features (DSC: median [IQR], 0.601 [0.494–0.712] and 0.651 [0.52–0.784] for lung and liver lesions, respectively) were more stable than those obtained with all features (DSC: median [IQR], 0.532 [0.424–0.637] and 0.587 [0.465–0.703] for lung and liver lesions, respectively; P <.001). In the case study, CT habitats correlated quantitatively and qualitatively with heterogeneity observed in multiparametric MRI habitats and histology. Conclusion: Precise three-dimensional radiomics features were identified on CT images that enabled tumor heterogeneity assessment through stable tumor habitat computation.