Objective- We evaluated the biological effects of low-dose methotrexate on 3 novel brachial artery grayscale ultrasound measures that may indicate subclinical arterial injury. Approach and Results- Exploratory analysis from a clinical trial of people with HIV infection at increased cardiovascular disease risk who were randomly assigned to low-dose methotrexate (target dose 15 mg/wk) or placebo. Brachial artery ultrasound grayscale median, gray level difference statistic texture-contrast (GLDS-CON), and gray level texture entropy were measured at baseline and after 24 weeks of intervention. Findings from the intention-to-treat (N=148) and adequately-dosed (N=118) populations were consistent, so the adequately-dosed population results are presented. Participants were a median (Q1, Q3) age of 54 (50, 60) years. After 24 weeks, the low-dose methotrexate intervention was associated with a 25.4% (-18.1, 58.6; P=0.007) increase in GLDS-CON compared with 1.3% (-29.1, 44.7; P=0.97) with placebo ( P=0.05) and a 0.10 u (-0.06, 0.23; P=0.026) increase in entropy compared with 0.02 u (-0.11, 0.14; P=0.54) with placebo ( P=0.14). At week 24, changes in CD4+ T cells correlated inversely with changes in GLDS-CON (ρ=-0.20; P=0.031), and entropy (ρ=-0.21; P=0.023). Changes in D-dimer levels, but no other inflammatory biomarkers, also correlated inversely with changes in GLDS-CON (ρ=-0.23; P=0.014) and entropy (ρ=-0.26; P=0.005). Conclusions- Brachial artery GLDS-CON and entropy increased after 24 weeks of low-dose methotrexate, though the latter was not significantly different from placebo. Grayscale changes were associated with decreases in CD4+ T-cell and D-dimer concentrations and may indicate favorable arterial structure changes.