Although constipation is one of the most frequently reported symptoms in palliative care, there is no widely accepted definition of constipation and none suitable for patients unable to self-report or express symptoms. This study aimed to verify the objective and subjective symptoms of constipation to develop a diagnostic algorithm for constipation, which is also feasible in unconscious patients. In a pooled analysis of two observational studies, 369 out of 547 adult end-stage cancer patients met the inclusion criteria. The patient-reported difficulty of defecation correlated with observable measures, such as days since last bowel movement, and frequency of bowel movements. Difficulty became at least moderate when there were no bowel movements for ≥2 days, or the frequency of bowel movements was ≤3 per week. The diagnostic algorithm, comprising these three symptoms offers a simple, rapid, and comprehensive tool for palliative care, independent of the patient's state of consciousness. A clinical trial is necessary to confirm its validity and usefulness.