<p>The gut microbiota-derived metabolite phenylacetylglutamine (PAGln) was associated with adverse events in patients with ST-segment elevation myocardial infarction (STEMI). However, the evidence regarding the association between PAGln and culprit plaque characteristics was limited. Here, we aim to investigate the associations between plasma PAGln and culprit plaque characteristics evaluated by optical coherence tomography (OCT) in patients with STEMI. The associations of PAGln level with culprit plaque characteristics were assessed with logistic regression model in 776 patients imaged by OCT at the time of primary percutaneous coronary intervention. The role of PAGln in plaque stability was investigated by bioinformation analysis. Patients in the high plasma PAGln group had a higher prevalence of culprit thin-cap fibroatheromas (TCFA) (37.4 vs. 30.5%, <i>p</i> = 0.001) and CLIMA-defined high risk plaque (37.1 vs. 30.2%, <i>p</i> = 0.001) compared with those in the low PAGln group. In the multivariate logistic analysis, elevated PAGln level remained independently (traditional risk factors) associated with TCFA (OR 1.95, 95% CI 1.29–2.95, <i>p</i> = 0.001) and CLIMA-defined high risk plaque (OR 1.99, 95% CI 1.31–3.02, <i>p</i> = 0.001). Single-cell RNA sequencing results suggested that PAGln-targeted <i>ACE</i>, <i>MME</i>, <i>MMP9</i> genes enrich macrophages and may be involved in plaque stability in STEMI patients. This study first revealed that PAGln concentrations were significantly associated with the vulnerable plaque independently, and macrophages may be involved in this process.</p> Graphical Abstract <p>PAGln phenylacetylglutamine, TCFA thin-cap fibroatheroma, STEMI ST-segment elevation myocardial infarction.</p> <p></p>