BACKGROUND: While the mean left atrial pressure (mLAP) reduction during mitral transcatheter edge-to-edge repair (M-TEER) has been suggested as a potential prognostic marker, comprehensive evidence from large-scale, real-world cohorts is limited, especially with stratification by mitral regurgitation (MR) aetiology. AIMS: This study aimed to evaluate the prognostic significance of intraprocedural haemodynamic changes, particularly the mLAP reduction, during M-TEER for MR, using multicentre registry data. METHODS: From the OCEAN-Mitral registry, 2,629 patients undergoing M-TEER with intraprocedural direct mLAP measurements were included. A decrease in the mLAP was defined as follows: postprocedural mLAP-preprocedural mLAP<0. The primary outcome was the composite of all-cause mortality and heart failure (HF) rehospitalisation. RESULTS: Among 2,629 patients (degenerative MR [DMR]: 825 [31.4%], functional MR [FMR]: 1,804 [68.6%]), the postprocedural mLAP decreased in 1,548 patients (58.9%). In the DMR cohort, the mLAP reduction was associated with a significantly lower risk of the composite outcome (adjusted hazard ratio [HR] 0.66, 95% confidence interval [CI]: 0.45-0.96; p=0.028), while no such association was observed in the FMR cohort (adjusted HR 0.90, 95% CI: 0.75-1.08; p=0.251). Interaction analysis confirmed a significant difference in prognostic impact based on MR aetiology (p for interaction=0.006). Pulmonary vein flow patterns improved, with systolic waves becoming greater than diastolic waves across both groups; however, their combination with the mLAP reduction predicted better outcomes only in DMR patients. CONCLUSIONS: A reduction in the mLAP during M-TEER is independently associated with improved clinical outcomes in patients with DMR but not in those with FMR. These findings underscore the importance of real-time haemodynamic assessment - especially in DMR - as a valuable procedural endpoint that may aid in optimising long-term outcomes beyond anatomical MR reduction alone.