The aim of the study was to assess dietary diversity (DD) and associated factors among human immune deficiency virus (HIV)-positive adults attending the anti-retroviral therapy (ART) clinic at Felege Hiwot Comprehensive Specialized Hospital (FHCSH) in Northwest Ethiopia. An institution-based cross-sectional study was conducted at FHCSH in 2019. A systematic random sampling technique was employed to select 352 study subjects. Data were collected using an interviewer-administered questionnaire and chart review. Statistical Package for the Social Science version 26 was used for analysis. A simple and multivariable binary logistic regression was used to determine associated factors. Two hundred and nine (59⋅4 %) adults had consumed a diversified diet. The mean individual DD score was 3⋅86 ± 1⋅18. Self-employment status (adjusted odd ratio (AOR): 4⋅60; 95 % confidence interval (CI): 1⋅72, 12⋅27), quintiles of wealth index (the second (AOR: 4⋅33; 95 % CI: 1⋅72, 10⋅89), middle (AOR: 4⋅40; 95 % CI: 1⋅71, 11⋅31), fourth (AOR: 6⋅60; 95 % CI: 2⋅36, 18⋅48) and the highest quintiles (AOR: 9⋅45: 95 % CI: 3⋅34, 26⋅77), the last CD4 count 200–349 cells/mm3 (AOR: 8⋅08; 95 % CI: 2⋅93, 22⋅23), those who took first-line ART regimen drugs (AOR: 4⋅49; 95 % CI: 2⋅19, 9⋅21), subjects who did not take co-trimoxazole prophylaxis (AOR: 6⋅36; 95 % CI: 2⋅54, 15⋅88), those who had nutritional counselling at a health institution (AOR: 2⋅36; 95 % CI: 1⋅08, 5⋅16), had no food preference (AOR: 2⋅42; 95 % CI: 1⋅14, 5⋅13) and a food-secure household (AOR: 3⋅51; 95 % CI: 1⋅85, 6⋅67) were associated factors of DD among adults on ART. This study exhibited that the DD status among adults attending the ART clinic was below two-thirds. Health institutions and health professionals working at ART clinics shall strengthen their efforts to sustain the nutritional counselling service and ART adherence at health institutions and encourage the patients to avoid food preference for their meal. It is vital to ensure the household food security of adults on ART.