Background: Cancer is one of the leading causes of death in the world and it is considered that every fourth person dies of it. undernutrition is most commonly seen in cancer patients with some types of solid tumors, various chronic diseases, as well as in older persons and young children. This can result in longer hospital stay, reduced response to therapies, increased complications to therapy and surgery proceedings, poor survival and higher care costs. Objective: The main aim of this research was to assess prevalence and factors associated with undernutrition on cancer patients Attending Tikur Anbessa Specialized Hospital, Ethiopia. Method: Cross sectional study was conducted from March to May/2018 among 347 cancer patients in Tikur Anbessa Specialized hospital Addis Ababa, Ethiopia. All cancer patients 18-65 years of age who were on 2nd cycle and above treatment phase in the study were included and patients who were seriously ill and unable to talk was excluded from the study. Quantitative data was collected using questionnaire and Patient Generated-Subjective Global Assessment score. Data was entered in to Epi-Info version 7 and exported to and analyzed by SPSS version 20. Both bivariate and multivariate logistic regression analyses were employed to identify the associated factors. Variables with value of less than 0.05 were considered as significant predictors. Result: The prevalence of undernutrition according to PG-SGA score result 202(63.1%) and 88(27.5%) of the participants were moderately and severely undernourished respectively. BMI of the participants also showed that 206(64.4%) and 89(27.8%) were normal and underweight respectively. Two hundred seventy six (86.3%) of the patients were in need of critical nutrition intervention. Performance status of ≥2 [AOR = 7.9, 95 % CI (3.05, 20.48)], cancer stage II, III & IV [AOR = 3.47, 95 % CI (1.25, 9.58)], [AOR = 3.81, 95 % CI (1.17, 12.31)] and [AOR = 6.11, 95 % CI (1.48, 25.14)] respectively factors associated with malnutrition on cancer patients. Conclusion and recommendations: The prevalence of undernutrition is prevalent in the study area therefore, there should be timely screening and evaluation of nutritional status of the patients and planning nutritional therapy such as dietary supplements or enteral nutrition to counteract malnutrition on cancer patients is recommended