While previous studies suggest a cardio-splenic axis involving splenic activation and systemic inflammation, presence and clinical significance of splenic tissue alterations on cardiac magnetic resonance (CMR) in ST-segment-elevation myocardial infarction (STEMI) remain unclear. This study aims to identify determinants of splenic T1 patterns in STEMI and explore their association with adverse outcomes. This retrospective study enrolled STEMI patients undergoing CMR within 10 days after primary percutaneous coronary intervention (April 2017-October 2024). CMR evaluated infarction patterns, myocardial function, native splenic T1 times and extracellular volume (ECV). Major adverse cardiovascular events (MACE) included cardiovascular death, heart failure and myocardial re-infarction. Results were validated in a separate cohort. Associations between variables and outcomes were evaluated using Cox regression. The development and validation cohorts comprised 1037 (median age 60; 872 men) and 252 (median age 59; 212 men) patients, respectively. Over a median follow-up of 38 and 30 months, 207 and 40 MACE occurred. In multivariable Cox regression, both elevated splenic T1 z-score (adjusted HR 1.58) and splenic ECV (adjusted HR 1.22) independently predicted MACE, driven primarily by heart failure and non-target vessel re-infarction. The model combining clinical and conventional myocardial imaging risk factors with splenic T1 z-score demonstrated best discrimination among all tested models (development C index=0.87; validation C index=0.85). Elevated splenic T1 z-score and ECV assessed by CMR after STEMI independently predicted MACE, mainly because of higher risk of heart failure and non-target vessel re-infarction, providing incremental prognostic value over traditional clinical and imaging risk markers.