Introduction <p>The relationship between maxillary posterior tooth roots and the maxillary sinus floor is a key risk factor in orthodontic treatment. This study systematically evaluated this relationship for the maxillary second premolar, first molar, and second molar in adults with different growing patterns.</p> Methods <p>This retrospective cross-sectional study analyzed CBCT data from 502 adults (20–45 years). Patients were grouped by sagittal pattern (ANB angle: Class I, II, III) and vertical skeletal pattern (MP-SN angle: hyperdivergent, normodivergent, hypodivergent). A modified classification (scores 1–5) assessed the vertical relationship between each tooth apex and the sinus floor. The effects of age, sex, and skeletal pattern on scores were analyzed.</p> Results <p>After adjusting for age and sex, the vertical pattern showed no significant association with root-sinus scores (P &gt; 0.05). Sagittally, Class II patients had significantly lower first molar scores than Class I patients (<i>P</i> = 0.032), indicating roots positioned farther from the sinus floor. Age was significantly negatively correlated with scores (P &lt; 0.001), and females had lower scores than males (<i>P</i> &lt; 0.001). While scores among tooth positions were correlated at the population level, high individual heterogeneity was observed, with 43.4% of patients showing irregular fluctuations.</p> Conclusions <p>Vertical skeletal pattern is not significantly associated with the maxillary posterior root-sinus relationship in adults. Skeletal Class II patients exhibit a more distant maxillary first molar apex-to-sinus adjacency, indicating a static anatomical association. Increasing age is associated with more distant root-to-sinus adjacencies.</p>