IntroductionBreast cancer is a chronic disease that has implications for many aspects of the patient's life. Contracting the COVID-19 virus places cancer patients at a higher risk of infection. This condition triggers uncertainty which causes emotional responses.ObjectiveThe aim of this study was to measure the relationship between the uncertainty perspective of breast cancer patients and emotional responses during the COVID-19 pandemic.MethodsThis study used an observational study with a cross-sectional design. Data was collected from May to December 2021. The total sample of this study was 110 breast cancer patients undergoing chemotherapy at the Army Hospital of Dr. Soepraeon Malang, Indonesia. We used purposive sampling. The questionnaire used was a modified questionnaire from the Mishel Uncertainty in Illness Scale, a modified questionnaire from the Concerns about Recurrence Questionnaire, a modified questionnaire from the Zung Self Rating Anxiety Scale, and a modified questionnaire from the Depression, Anxiety, and Stress Scale. Data was analyzed using SPSS with a Spearman correlation test.ResultsThe mean uncertainty of the respondents was 75.98 or in the moderate category, emotional response was moderate fear with a mean score of 18.40, the average anxiety score was 41.05 or normal, and the mean depression score was 15.96 or low depression. In addition, there was a significant relationship between uncertainty and the emotional response among breast cancer patients in the era of the COVID-19 pandemic ( p < .05).ConclusionThis study showed that there was a relationship between uncertainty and emotional response among breast cancer patients. It is important for nurses to provide good information about the disease among patients by using therapeutic communication and paying attention to the negative emotional responses of breast cancer patients.