Introduction: Patients with HIV/AIDS are at high risk of consuming low-quality, monotonous food. The current study aimed to assess dietary intake adequacy and its associated factors among adult ART patients in Ilu Abbabor Zone, South West, Ethiopia. Methods: A facility-based cross-sectional study was conducted in six different health care facilities. Epi-Data was used for data entry, and SPSS version 21 was used for analysis. The level of significance in the final model was set at (P < 0.05). Results: The current study found that the majority of respondents (61.2%) had an inadequate dietary diversity practice. Adults on ART, on the other hand, have adequate dietary diversity practice. On the other hand, dietary diversity practice of adult people on ART, significantly associated with; being ever married [AOR = 0.366; 95% CI: (0.211, 0.634)], household number >/=5 [AOR = 2.45; 95% CI: (1.522, 4.088)], clinical stage of disease [AOR = 2.474; 95% CI (1.124, 5.444), and obesity [AOR = 0.290; 95% CI: (0.099, 0.846)]. Conclusions: The vast majority of study subjects (61.2%) had an inadequate dietary diversity practice. Furthermore, the adequacy of adult ART dietary intake was significantly related to marital status, household number, clinical stage of disease, and nutritional status. As a result, unmarried adult people living with HIV/AIDS must receive care and support. It is strongly advised to integrate nutrition with RH services, particularly family planning. Finally, early HIV/AIDS diagnosis and management, as well as nutrition, are critical for delaying the rapid progression of HIV to Acquired Immune Deficiency Syndrome (AIDS).