BACKGROUND: m6A RNA methylation is a critical epigenetic modification involved in the pathogenesis of pulmonary arterial hypertension (PAH). While macrophage-mediated inflammation plays a central role in PAH, the specific contributions of m6A regulators within macrophages are not yet fully understood. This study explores the role of METTL3 in macrophages, with particular emphasis on its contribution to the progression of PAH. METHODS: SU5416/Hypoxia (SuHx) PAH mouse models were treated daily with STM2457, a selective METTL3 antagonist, or vehicle for 10 days. Additionally, SuHx PAH was induced in Mettl3flox/floxlyz2cre + and Mettl3flox/flox mice using genetic approaches. Pulmonary acceleration time to pulmonary ejection time (PAAT/PAET) and right ventricular free wall (RVFWD) were measured by ultrasound. Hemodynamic parameters, including right ventricular systolic pressure (RVSP), were assessed. Pulmonary vascular and right heart remodeling were evaluated using HE staining, while vascular and right heart fibrosis were assessed by Masson's trichrome staining. The expression of fibrosis-associated genes was quantified by qPCR. Macrophage activation in tissues was determined via CD86 and CD206 immunofluorescence staining, and the expression of inflammatory cytokines and fibrosis-associated genes was quantified by qPCR. RESULTS: METTL3 expression was significantly upregulated in the lungs and macrophages of PAH models. Treatment with STM2457 reversed the progression of SuHx PAH, as evidenced by a reduction in RVSP, attenuation of pulmonary vascular and right heart remodeling, and decreased fibrosis in both the heart and lungs. Furthermore, the expression of fibrosis-associated genes in the right heart, including Col1a1, Col1a3, and α-SMA, was downregulated following STM2457 treatment and METTL3 depletion in macrophages. Both STM2457 treatment and METTL3 depletion resulted in a significant reduction in the number of CD86 CONCLUSION: STM2457 treatment and METTL3 depletion in macrophages effectively reversed SuHx PAH by modulating macrophage inflammatory responses and alleviating pulmonary vascular and right heart remodeling, as well as fibrosis. These findings underscore the role of METTL3 in PAH pathogenesis by regulating macrophage function and inflammation.