The goal of our study was to investigate the heterogeneity of cardiac macrophages (CMφs) in mice with transverse aortic constriction (TAC) via single-cell sequencing and identify a subset of macrophages associated with heart injury.We selected all CMφs from CD45+ cells using single-cell mRNA sequencing data. Through dimension reduction, clustering and enrichment analyses, CD72hi CMφs were identified as a subset of proinflammatory macrophages. The pseudotime trajectory and ChIP-Seq analyses identified Rel as the key transcription factor that induces CD72hi CMφ differentiation. Rel KD and Rel-/- bone marrow chimera mice subjected to TAC showed features of mitigated cardiac injury, including decreased levels of cytokines and ROS, which prohibited cardiomyocyte death. The transfer of adoptive Rel-overexpressing monocytes and CD72hi CMφ injection directly aggravated heart injury in the TAC model. The CD72hi macrophages also exerted proinflammatory and cardiac injury effects associated with myocardial infarction (MI). In humans, patients with heart failure exhibit increased CD72hi CMφ levels following dilated cardiomyopathy (DCM) and ischemic cardiomyopathy (ICM).Bone marrow-derived, Rel-mediated CD72hi macrophages play a proinflammatory role, induce cardiac injury and, thus, may serve as a therapeutic target for multiple cardiovascular diseases.Heart failure (HF) imposes an enormous clinical and economic burden worldwide and presents limited therapeutic approaches. Given the close association between inflammation and adverse outcomes, proinflammatory immune cells are considered potential therapeutic targets for HF treatment. The present studies identified a specific macrophage subset associated with myocardial injury, which may provide an alternative approach for treating cardiovascular diseases.Published on behalf of the European Society of Cardiology. All rights reserved. © The Author(s) 2021. For permissions please email: journals.permissions@oup.com.