Background We aimed to investigate novel grayscale ultrasound characteristics of the carotid and brachial arteries in people with HIV infection before and after starting initial antiretroviral therapy (ART). Methods and Results We performed grayscale ultrasound image analyses of the common carotid artery (CCA) and brachial artery before and after receipt of 1 of 3 randomly allocated ART regimens. We measured arterial wall echogenicity (grayscale median), contrast (gray‐level difference statistic method), and entropy. These measures and their changes were compared with atherosclerotic cardiovascular disease risk factors, measures of HIV disease severity, and inflammatory biomarkers before and after ART. Changes in the grayscale measures were evaluated within and between ART arms. Among 201 ART‐naïve people with HIV, higher systolic blood pressure, higher body mass index, lower CD4+ T cells, and non‐Hispanic White race and ethnicity were associated independently with lower CCA grayscale median. Changes in each CCA grayscale measure from baseline to 144 weeks correlated with changes in soluble CD163: grayscale median (ρ=−0.17; P =0.044), gray‐level difference statistic–contrast (ρ=−0.19; P =0.024), and entropy (ρ=−0.21; P =0.016). Within the atazanavir/ritonavir arm, CCA entropy increased (adjusted β=0.023 [95% CI, 0.001–0.045]; P =0.04), but no other within‐arm changes in grayscale measures were seen. Correlations of brachial artery grayscale measures were weaker. Conclusions In ART‐naïve people with HIV, CCA grayscale ultrasound measures were associated with atherosclerotic cardiovascular disease risk factors and lower grayscale median was associated with lower CD4+ T cells. Reductions in soluble CD163 with initial ART were associated with improvements in all 3 CCA grayscale measures, suggesting that reductions in macrophage activation with ART initiation may lead to less arterial injury. Registration URL: https://clinicaltrials.gov/ ; Unique identifiers: NCT00811954; NCT00851799