Perivascular adipose tissue (PVAT) is increasingly recognized as an essential layer of the functional vasculature, being responsible for producing vasoactive substances and assisting arterial stress relaxation. Here, we test the hypothesis that PVAT reduces aortic stiffness. Our model was the thoracic aorta of the male Sprague-Dawley rat. Uniaxial mechanical tests for three groups of tissue were performed: aorta with PVAT attached (+PVAT) or removed (-PVAT), and isolated PVAT (PVAT only). The output of the mechanical test is reported in the form of a Cauchy stress-stretch curve. This work presents a novel, physiologically relevant approach to measure mechanical stiffness ex vivo in isolated PVAT. Low-stress stiffness ( ), high-stress stiffness ( ), and the stress corresponding to a stretch of 1.2 (σ ) were measured as metrics of distensibility. The low-stress stiffness was largest in the -PVAT samples and smallest in PVAT only samples. Both the high-stress stiffness and the stress at 1.2 stretch were significantly higher in -PVAT samples when compared with +PVAT samples. Taken together, these results suggest that -PVAT samples are stiffer (less distensible) both at low stress (not significant) as well as at high stress (significant) when compared with +PVAT samples. These conclusions are supported by the results of the continuum mechanics material model that we also used to interpret the same experimental data. Thus, tissue stiffness is significantly lower when considering PVAT as part of the aortic wall. As such, PVAT should be considered as a target for improving vascular function in diseases with elevated aortic stiffness, including hypertension. We introduce a novel and physiologically relevant way of measuring perivascular adipose tissue (PVAT) mechanical stiffness which shows that PVAT's low, yet measurable, stiffness is linearly correlated with the amount of collagen fibers present within the tissue. Including PVAT in the measurement of the aortic wall's mechanical behavior is important, and it significantly affects the resulting metrics by decreasing aortic stiffness.