Objectives <p>This study aimed to investigate the association between the triglyceride-glucose index (TyG) and Short-Term Prognosis in patients with acute myocarditis, addressing the knowledge gap on the TyG index in this patient cohort.</p> Methods <p>In this cohort study, clinical data were retrospectively gathered from adults with acute myocarditis across three hospital districts—Zhongfa District, Guanggu District, and Qiaokou District of Tongji Hospital. The primary outcome measured was In-hospital Mortality. Non-linear relationships were assessed through restricted cubic spline (RCS) regression and U-test analyses. The impact of the TyG index on prognosis was evaluated using generalized linear models with a logit link function. Subgroup analyses were conducted across different age, gender, smoking, drinking, and comorbidity categories.</p> Results <p>A total of 1,373 patients were included in the study. Various cardiac morphological and functional indices, such as pericardial effusion, ventricular wall motion abnormalities, atrial enlargement, mitral stenosis/insufficiency, tricuspid stenosis/insufficiency, left ventricular systolic dysfunction, and pleural effusion, exhibited significant changes with increasing TyG index levels (all <i>P</i> &lt; 0.05). The TyG index emerged as an independent risk factor for In-hospital Mortality among acute myocarditis patients (adjusted odds ratio (OR) 2.266 (95% CI: 1.681–3.055), <i>P</i> &lt; 0.001), and RCS regression demonstrated a non-linear relationship with In-hospital Mortality (<i>P</i> for non-linearity &lt; 0.001). Elevated or reduced TyG levels were associated with increased In-hospital Mortality (U-test <i>P</i> &lt; 0.05). Subgroup analyses found no interaction effect in the correlation between the TyG index and In-hospital Mortality.</p> Conclusions <p>Extremes in TyG levels, whether high or low, are linked to elevated In-hospital Mortality in patients with acute myocarditis.</p>