Any person in any career may experience stress, which is a common negative emotional feeling that affects people of all ages worldwide. It is commonly known that university undergraduate students worldwide have a high prevalence of stress. Students from various socioeconomic backgrounds who enter universities may have mental health issues. There is a shortage of studies on this topic in the Sri Lankan setting, and it is of great importance to identify the factors affecting stress among undergraduate students. Therefore, the main objective of this study was to identify the factors affecting stress among undergraduate students. A sample of 384 undergraduate students from the Sabaragamuwa University of Sri Lanka was selected using the two-stage cluster sampling technique. The primary data was collected through a structured questionnaire. The questionnaire included the DASS-21 scale (Depression, Anxiety, and Stress Scale), which measured the undergraduates level of stress. Binary logistic regression was performed with 23 independent variables to accomplish the main goal. The pressure of the thesis and internship and financial difficulties were found to have a positive effect on undergraduates stress. The pressure of the thesis and internship and a lack of sleep were found to be significant determinants for female students. Male students stress was found to be influenced by the pressure of thesis and internship, lack of sleep, and subject difficulties. For students, university administrators, lecturers, and policymakers, the whole study provides significant recommendations on how to reduce stress.