Purpose <p>Metabolic dysfunction–associated steatotic liver disease (MASLD) and non-alcoholic fatty pancreatic disease (NAFPD) are metabolic diseases with rising incidence. Fatty infiltration may lead to dysfunction of the liver and pancreatic tissues. This study aims to quantify liver and pancreatic fat fractions and examine their correlation with disease severity in acute pancreatitis patients.</p> Methods <p>The severity of acute pancreatitis was assessed using the revised Atlanta classification (RAC), computed tomography severity index (CTSI), and modified CTSI (mCTSI). Proton density fat fraction (PDFF) levels of the liver and pancreas were measured via IDEAL MRI. Patients were categorized into biliary and non-biliary pancreatitis groups. Correlations between PDFF levels and the RAC, CTSI, and mCTSI scores were analyzed.</p> Results <p>A total of 127 patients were included, with MASLD present in 40.9% and NAFPD in 30%. Liver PDFF values were significantly higher in non-biliary pancreatitis (<i>p</i> = 0.040). Patients with MASLD exhibited higher CTSI and mCTSI scores (<i>p</i> = 0.009, <i>p</i> = 0.033, respectively). No significant differences were observed in severity scales between patients with and without NAFPD. Liver PDFF was positively correlated with CTSI and mCTSI scores in biliary pancreatitis. ROC analysis identified a liver PDFF &gt; 3.9% (<i>p</i> = 0.002) and pancreatic corpus PDFF &gt; 12.1% (0.028) as diagnostic markers for severe pancreatitis. In addition, a liver PDFF &lt; 4.5% (<i>p</i> = 0.042) was an indicator for biliary pancreatitis.</p> Conclusion <p>MASLD is associated with increased severity in acute pancreatitis. IDEAL MRI-derived PDFF levels of the liver and pancreas show potential in predicting severe acute pancreatitis and distinguishing between biliary and non-biliary etiologies.</p>