Background: HIV/AIDS is associated with an increased burden of undernutrition among children even under antiretroviral therapy (ART). To treat undernutrition, WHO endorsed the use of Ready to Use Therapeutic Foods (RUTF) that can reduce case fatality and undernutrition among ART-naïve HIV-positive children. However, its effects are not studied among ART-treated, HIV-positive children. Therefore, we examined the association between RUTF use with underweight, wasting, and stunting statuses among ART-treated HIV-positive children in Dar es Salaam, Tanzania. Methods: This cross-sectional study was conducted from September-October 2010. The target population was 219 ART-treated, HIV-positive children and the same number of their caregivers. We used questionnaires to measure socio-economic factors, food security, RUTF-use, and ART-duration. Our outcome variables were underweight, wasting, and stunting statuses. Results: Of 219 ART-treated, HIV-positive children, 140 (63.9%) had received RUTF intervention prior to the interview. The percentages of underweight and wasting among non-RUTF-receivers were 12.4% and 16.5%; whereas those of RUTF-receivers were 3.0% (P = 0.006) and 2.8% (P = 0.001), respectively. RUTF-receivers were less likely to have underweight (Adjusted Odd Ratio (AOR) =0.19, CI: 0.04, 0.78), and wasting (AOR = 0.24, CI: 0.07, 0.81), compared to non RUTF-receivers. Among RUTF receivers, children treated for at least four months (n = 84) were less likely to have underweight (P = 0.049), wasting (P = 0.049) and stunting (P < 0.001). Conclusions: Among HIV-positive children under ART, the provision of RUTF for at least four months was associated with low proportions of undernutrition status. RUTF has a potential to improve undernutrition among HIV-positive children under ART in the clinical settings in Dar es Salaam, Tanzania. Methods: This cross-sectional study was conducted from September-October 2010. The target population was 219 ART-treated, HIV-positive children and the same number of their caregivers. We used questionnaires to measure socio-economic factors, food security, RUTF-use, and ART-duration. Our outcome variables were underweight, wasting, and stunting statuses. Results: Of 219 ART-treated, HIV-positive children, 140 (63.9%) had received RUTF intervention prior to the interview. The percentages of underweight and wasting among non-RUTF-receivers were 12.4% and 16.5%; whereas those of RUTF-receivers were 3.0% (P = 0.006) and 2.8% (P = 0.001), respectively. RUTF-receivers were less likely to have underweight (Adjusted Odd Ratio (AOR) =0.19, CI: 0.04, 0.78), and wasting (AOR = 0.24, CI: 0.07, 0.81), compared to non RUTF-receivers. Among RUTF receivers, children treated for at least four months (n = 84) were less likely to have underweight (P = 0.049), wasting (P = 0.049) and stunting (P < 0.001). Conclusions: Among HIV-positive children under ART, the provision of RUTF for at least four months was associated with low proportions of undernutrition status. RUTF has a potential to improve undernutrition among HIV-positive children under ART in the clinical settings in Dar es Salaam, Tanzania.