Context Undertreatment of cancer pain among outpatient cancer patients needs to be addressed to enhance care and improve patients' quality of life (QoL). Objectives This prospective, cross-sectional, patient-focused study aimed to explore the prevalence of pain and undertreatment of cancer pain in outpatients in Taiwan. Methods A total of 2652 non-selected outpatients with cancer and aged 20 years or older from 16 medical centers across Taiwan were included in this survey. All patients completed a questionnaire based on the Brief Pain Inventory. Pain management index (PMI) was used to evaluate the adequacy of pain management. Possible clinical variables of patients with positive PMI were examined by univariate and multivariate logistic regressions. Results A total of 1659 (62.6%) outpatients had experienced some degree of pain; among these, 32.4% had negative PMI. Patients with a negative PMI score had significantly poor outcomes of QoL and a significantly higher tendency toward dissatisfaction with pain control by the physician and with the prescribed analgesic drugs. Female gender, primary tumor from breast, non–cancer-related cause of pain, and hospital locations from north Taiwan were independent variables that predicated patients with undertreatment of cancer pain. Most importantly, a forward trend of undertreatment of pain among patients who presented with lower prevalent rate of pain was observed. Conclusion One-third of Taiwanese outpatients experienced pain because of undertreatment. Awareness of the prevalence of undertreatment of cancer pain and identification of the vulnerable subjects may assist in enhancing patient care and improving patient's QoL.