Background: Cancer has become a significant public health issue around the world and an increasingly important contributor to disease burdens. In countries like Ethiopia with high nutrient demands, people with chronic diseases like cancer are at a high risk of macro and micronutrient deficiencies. Therefore, the present study attempted to assess dietary diversity and associated factors among adult cancer patients attending treatment at Black Lion Specialized Hospital, Addis Ababa, Ethiopia. Method and Materials: Hospital-based cross-sectional study was conducted from 22 April 2021 to 22 May 2021 on 416 adult cancer patients at Black Lion Specialized Hospital (BLSH). A systematic random sampling technique was applied to select study subjects. Quantitative data were collected using a structured, pretested and interviewer-administered questionnaire. The questionnaire comprised the standard dietary diversity measurement tool, which was adopted from the Food and Technical Assistance (FANTA) then data were entered into EPI INFO software and analysed using Statistical Package for the Social Sciences (SPSS) version 25. Frequency, mean and standard deviation were used to describe variables. A binary logistic regression model was fitted to elicit factors associated with the dietary diversity of cancer patients and a P-value of less than 0⋅2 was used as a cut-off for further analysis. Logistic regression analysis with a 95 % confidence interval (CI) was estimated to measure the strength of association at P < 0⋅05. Results: The present study revealed that 61⋅5 % of patients had low dietary diversity. Being from a family size of five and more (AOR = 1⋅48, 95 % CI 1⋅28, 1⋅83), having no permanent income (AOR = 1⋅31, 95 % CI 1⋅15, 1⋅67), alcohol consumption (AOR = 3⋅97, 95 % CI 1⋅20, 13⋅1), not doing regular physical exercise (AOR = 1⋅83, 95 % CI 1⋅07, 3⋅12), lack of nutritional information (AOR = 2⋅23, 95 % CI 1⋅30, 3⋅82), poor nutritional knowledge (AOR = 1⋅84, 95 % CI 1⋅05, 3⋅25) and minimum meal frequency (AOR = 10⋅7, 95 % CI 5⋅04, 22⋅7) were factors associated with inadequate dietary diversity. Conclusion: The present study showed that the majority of cancer patients had low dietary diversity, suggesting that they are highly vulnerable to micronutrient deficiencies. Therefore, efforts should be strengthened to improve patients’ income level, access to nutrition information and nutritional knowledge.