Background: Immune checkpoint inhibitor (ICI)-related myocarditis is a rare but severe side effect that requires attention to distinguish it from other cardiovascular emergencies. Therefore, new biomarkers are needed for the early diagnosis of ICI-related myocarditis, especially to distinguish it from other cardiovascular emergencies. Our study aims to explore plasma biomarkers with high specificity and sensitivity using proteomic analysis for the early diagnosis of ICI-related myocarditis and to distinguish it from acute myocardial infarction (AMI). Methods: Patients with ICI-related myocarditis and AMI were selected and matched for potential confounding effects such as age, gender, smoking history, and chronic heart diseases. A label-free liquid chromatography-mass spectrometry (LC-MS) proteomics approach was used to analyze plasma samples and screen sensitive and specific myocarditis protein biomarkers. Further validation will be performed at the level of patients and animal models of ICI-related myocarditis and AMI. Results: We performed plasma proteomics on 15 plasma samples from 5 pairs of samples with PD-1 inhibitor-associated myocarditis at baseline (control group) and diagnosis (myocarditis group) and 5 cases of acute myocardial infarction (MI group) confirmed by coronary angiography in the Second Affiliated Hospital of Nanchang University. In total, we identified a total of 1521 plasma proteins and 1325 quantifiable plasma proteins across all 15 plasma samples. Further analysis of the differential proteins showed that 34 proteins with significantly differentially expressed in myocarditis group samples compared with other groups. Among them, several proteins were significantly up-regulated (MYOM3, TNNC1, MYL1, MYOM2, CA3, MYLPF, ECI1, LGALS1, PSMA7, PSMB3, PSMA4, PSMB6, CKM, ABCB6, UGP2, ALDOA, TPM2) or down-regulated (MAN1A1, SPTB, YTHDC2, CLEC11A, GSN, APOA4) in myocarditis samples. Compared with the other groups, the ICI-related myocarditis group showed molecular changes in myocardial contraction, proteasome, immunoregulation, NF-κB signaling pathway, arginine and proline metabolism, and cysteine and methionine metabolism. We then performed ELISA on additional ICI-related myocarditis and AMI patients and corresponding animal models. The Troponin C, Myom 3, UGP2, and Galectin-1 proteins related to immune or inflammation were significantly increased. Conclusions: Our study has revealed that proteins related to the immune system or inflammation may be sensitive and specific plasma biomarkers for identifying ICI-related myocarditis. However, further large sample cohorts are needed to confirm these findings.