Background: Ascending thoracic aortic dilation is a complex trait that involves modifiable and non-modifiable risk factors and can lead to thoracic aortic aneurysm and dissection. Clinical risk factors have been shown to predict ascending thoracic aortic diameter. Polygenic risk scores (PRS) are increasingly used to assess clinical risk for multifactorial diseases. The degree to which a PRS improves aortic diameter risk prediction is not known. In this study we tested the extent to which the addition of PRS to clinical prediction algorithms improves the prediction of aortic diameter. Methods: Training and validation cohorts comprised 6,790 Penn Medicine Biobank (PMBB) participants with clinical data linked to whole exome sequencing. Linear regression models integrated PRS weights derived from a large genome wide association study of thoracic aortic diameter in the UK biobank and were compared to the performance of the standard and a reweighted variation of the recently published AORTA Score. Results: The training cohort of 5,092 participants (56% male) had a median age of 61 years (IQR 52-70) with a mean ascending aortic diameter of 3.4 cm (SD 0.5). Compared to the AORTA Score which explained 29.1% (95% CI 27.9%-30.8%) of the variance in aortic diameter, AORTA Score + PRS (explained 29.7%, 95% CI 28.5%-30.9%), the reweighted AORTA score (explained 30.7%, 95% CI 29.4%-32.3%), and the reweighted AORTA Score + PRS (explained 31.5%, 95% CI 30.0%-32.8%) had superior performance. For the predictive ability to identify thoracic aortic diameter of ≥ 4 cm, the addition of a PRS to both the AORTA Score and the reweighted AORTA Score improved sensitivity at a range of model threshold values. The respective areas under the receiver operator characteristic curve for the AORTA Score + PRS (0.762, 95% CI 0.731-0.794) and reweighted AORTA Score + PRS (0.779, 95% CI 0.749-0.809) were greater than the standard AORTA Score (0.758, 95% CI 0.727-0.789) and reweighted AORTA Score (0.775, 95% CI 0.745-0.805). Conclusions: We demonstrated that inclusion of a PRS to the AORTA Score results in a small performance enhancement. Further investigation is necessary to determine if incorporation of genetics into clinical risk prediction improve outcomes for thoracic aortic disease.