Background: Regulatory T cells (Tregs) play a crucial role in the pathophysiology of abdominal aortic aneurysms (AAA), a chronic inflammatory condition with few treatment options for patients with early-stage disease. Treg therapy for AAA is potentially beneficial but its specific mechanism requires further investigation for clinical applications. Methods: After identifying the critical role of T-cells in AAA using human and mouse AAA single-cell RNA sequencing data, we investigated the influence of Tregs on immune cell infiltration within mouse AAA-specifically CD3+ T cells-using congenic transfer of Thy1.1 allelic donor mice Tregs into AAA-induced wild-type C57BL/6J mice. AAA progression was quantified with ultrasound and image micrometry. Tissues obtained on postoperative days 7-42 were analyzed with flow cytometry, qRT-PCR, Verhoeff-van Gieson staining, hematoxylin-eosin staining, and immunohistochemistry. Results: CD3+ T cell population was profoundly elevated in the elastase induced AAA mouse model which was further used in the study. The AAA mice that received Treg cell therapy had less elastin degradation and aortic wall enlargement than their control counterparts. Donor Tregs were detected in draining lymph nodes even after five weeks, with characteristic expression of FOXP3 and CD25. Although donor Tregs were not detected in the aortic microenvironment, the pro-inflammatory cell population including CD4 and CD8 cells was reduced, compared to control mice. Conclusion: Elevated T cell population aggravates inflammation and promotes AAA progression. Treg therapy impedes the recruitment of T cells into AAA tissue by colonizing the draining lymph nodes, thereby mitigating AAA progression. This study deepens our understanding of Treg stability, function, and potential as a promising therapy for early-stage aneurysms.