Background. HIV/AIDS and malnutrition are interrelated and exacerbate one another in a vicious cycle. As HIV infection progresses it causes catabolic state and increases susceptibility to other infections, leading to progressive aggravation of undernutrition. However, data are lacking in Ethiopia on determinants of undernutrition among people living with HIV on antiretroviral therapy. Therefore, this study aimed to assess determinants of undernutrition among adult HIV/AIDS patients in Northern Ethiopia. Method. Facility-based unmatched case-control study was conducted among 324 randomly selected people living with HIV on antiretroviral therapy (ART). A structured and pretested interviewer questionnaire was used to collect data, while digital Seca weight and Seca measuring rod were used to measure weight and height, respectively. Logistic regression was used to identify independent factors of undernutrition, and p value <0.05 was declared for statistical significance. All statistical analyses were performed using SPSS 21™. Result. This study revealed that people of younger age and those on ART (AOR = 0.29 (95% CI: 0.10, 0.84)) had low risk of being undernourished. However, average individual monthly income (AOR = 2.61 (95% CI: 1.48, 4.61)), not receiving nutritional counseling during visits (AOR = 2.5 (95% CI: 1.52–3.89)), and low diet diversity (AOR = 10.55 (95% CI: 4.17, 26.73)) had higher odds of undernutrition among people living with HIV/AIDS. Conclusion. Age of patients, average monthly income, nutritional counseling during visits, and diet diversity were the independent factors of undernutrition. Counseling on well-timed and sufficient consumption of nutritious foods, economic strengthening, and livelihood activities is important. Future longitudinal study is necessary to elucidate the problem of undernutrition among people living with HIV/AIDS.