Background: Cardiac macrophages (cMPs) are increasingly recognized as important regulators of myocardial homeostasis and disease, yet the role of noncoding RNA in these cells is largely unknown. Small RNA sequencing of the entire miRNomes of the major cardiac cell fractions revealed microRNA-21 (miR-21) as the single highest expressed microRNA in cMPs, both in health and disease (25% and 43% of all microRNA reads, respectively). MiR-21 has been previously reported as a key microRNA driving tissue fibrosis. Here, we aimed to determine the function of macrophage miR-21 on myocardial homeostasis and disease-associated remodeling. Methods: Macrophage-specific ablation of miR-21 in mice driven by Cx3cr1-Cre was used to determine the function of miR-21 in this cell type. As a disease model, mice were subjected to pressure overload for 6 and 28 days. Cardiac function was assessed in vivo by echocardiography, followed by histological analyses and single-cell sequencing. Cocultures of macrophages and cardiac fibroblasts were used to study macrophage-to-fibroblast signaling. Results: Mice with macrophage-specific genetic deletion of miR-21 were protected from interstitial fibrosis and cardiac dysfunction when subjected to pressure overload of the left ventricle. Single-cell sequencing of pressure-overloaded hearts from these mice revealed that miR-21 in macrophages is essential for their polarization toward a M1-like phenotype. Systematic quantification of intercellular communication mediated by ligand-receptor interactions across all cell types revealed that miR-21 primarily determined macrophage-fibroblast communication, promoting the transition from quiescent fibroblasts to myofibroblasts. Polarization of isolated macrophages in vitro toward a proinflammatory (M1-like) phenotype activated myofibroblast transdifferentiation of cardiac fibroblasts in a paracrine manner and was dependent on miR-21 in cMPs. Conclusions: Our data indicate a critical role of cMPs in pressure overload–induced cardiac fibrosis and dysfunction and reveal macrophage miR-21 as a key molecule for the profibrotic role of cMPs.