Background: Paracentesis is commonly undertaken in patients with cancer-related ascites. Aim: To systematically investigate the symptomatic benefits and harms experienced by patients with cancer undergoing paracentesis using real-world data in the palliative care setting. Design: Prospective, multisite, observational, consecutive cohort study. Benefits and harms of paracentesis were assessed between 01/07/2018 and 31/02/2021 as part of routine clinical assessments by treating clinicians at four timepoints: (T0) before paracentesis; (T1) once drainage ceased; (T2) 24 h after T1 and (T3) 28 days after T1 or next paracentesis, if sooner. Setting/participants: Data were collected from 11 participating sites across five countries (Australia, England, Hong Kong, Malaysia and New Zealand) on 111 patients undergoing paracentesis via a temporary (73%) or indwelling (21%) catheter: 51% male, median age 69 years, Australia-modified Karnofsky Performance Score 50. Results: At T1 (n = 100), symptoms had improved for most patients (81%), specifically abdominal distension (61%), abdominal pain (49%) and nausea (27%), with two-thirds experiencing improvement in ���2 symptoms. In the remaining patients, symptoms were unchanged (7%) or worse (12%). At least one harm occurred in 32% of patients, the most common being an ascitic leak (n = 14). By T3, 89% of patients had experienced some benefit and 36% some harm, including four patients who experienced serious harm, one of which was a fatal bowel perforation. Conclusion: Most patients obtained rapid benefits from paracentesis. Harms were less frequent and generally mild, but occasionally serious and fatal. Our findings help inform clinician-patient discussions about the potential outcomes of paracentesis in this frail population.