Abstract Background Percutaneous left atrial appendage closure (LAAC) is an emerging alternative to oral anticoagulation for stroke prevention in atrial fibrillation (AF) in patients with AF, elevated stroke risk and contraindications to long-term anticoagulation treatment. Optimal pre-procedural planning is essential to ensure optimal procedural results. Case summary We report the case of a 62-year-old man with a history of right cerebellar haematoma referred for LAA closure. We describe the first use of FEops HEARTguide patient-specific computational simulation in the planning of LAAC with the Watchman Flex device (Boston Scientific, Marlborough, MA, USA) in an unusual ‘whale tail’-like LAA anatomy. Discussion Percutaneous left atrial appendage (LAA) closure is feasible in the majority of patients. However, certain LAA anatomies may pose substantial technical challenges. This case shows the crucial role of a pre-procedural assessment based on patient-specific computational simulations for LAA closure in difficult scenarios resulting in a more efficient procedure with the optimal result and good clinical outcomes.