Background: Obesity in adolescents is increasing worldwide and associated with an elevated cardiovascular risk later in life. In a group-comparative study, we investigated the association between adiposity in adolescents and signs of vascular aging and inflammation. Methods: Thirty-nine adolescents (10–18 years old), 19 with obesity and 20 with normal weight, were enrolled. The intima thickness and intima/media thickness ratio (I/M) were assessed using high-resolution ultrasound in the common carotid artery (center frequency 22 MHz) and the distal radial artery (RA; 50 MHz). Increased intima and high I/M are signs of vascular aging. Body characteristics, high-sensitivity C-reactive protein (hs-CRP), plasma lipids, and glycemic parameters were measured. Results: Adolescents with obesity, compared to normal-weight peers, had elevated plasma lipid, insulin c-peptide, and hs-CRP levels, the latter increasing exponentially with increasing adiposity. Obese adolescents had a thicker RA intima layer [0.005 mm; 95% confidence intervals (0.000, 0.009); P = 0.043] and a higher RA I/M [0.10; (0.040, 0.147); P < 0.0007]. Group differences for the RA I/M remained significant after adjustment for age, sex, fasting plasma insulin, and body mass index, both separately and together (P = 0.032). The RA I/M was correlated with hs-CRP, and both were correlated with the analyzed cardiovascular risk factors. Receiver operating curve c-values for RA I/M (0.86) and hs-CRP (0.90) strongly indicated correct placement in the obese or non-obese group. Conclusions: Adolescents with obesity had significantly more extensive vascular aging in the muscular RA, than normal-weight peers. The findings support an inflammatory link between obesity and vascular aging in adolescents.