Background/Objectives: The local mechanical properties of periodontal supporting tissues influence their responses to orthodontic forces. This exploratory study sought to evaluate site-specific nanoindentation hardness in human root cementum and adjacent alveolar cortical bone and examine its association with cone-beam computed tomography (CBCT)- and micro-computed tomography (micro-CT)-derived relative imaging indices. Methods: Specimens from seven human cadavers were analyzed. Indentation hardness (HIT) was measured in the central and apical regions of root cementum and adjacent alveolar cortical bone. CBCT gray values and micro-CT-derived relative imaging indices were obtained from anatomically matched regions without phantom-based mineral-density calibration. Nonparametric tests, multiple linear regression analyses, and Spearman’s rank correlation analyses were performed. Results: Root cementum exhibited significantly higher HIT in the central region than in the apical region (p < 0.001). The central region and mandibular location were independently associated with higher cementum HIT. No significant regional variation or associated factors were identified for alveolar cortical bone. Neither CBCT nor micro-CT-derived indices were significantly associated with local tissue hardness. Conclusions: Human root cementum exhibits site-specific variation in hardness, unlike the adjacent alveolar cortical bone. Uncalibrated CT-derived relative imaging indices may not be significantly associated with local tissue hardness.