Abstract: Myocardial fibrosis is a major pathogenic factor contributing to cardiac remodeling and heart failure. Recent research has indicated that microRNAs play a crucial role in the progression of cardiac fibrosis. Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) have been shown to alleviate myocardial fibrosis by inhibiting the transforming growth factor β1 (TGF-β1) signaling pathway. Therefore, the current study aimed to elucidate the post-transcriptional regulation of BAMBI by miR-19a-3p and its role in TGF-β1-induced cardiac fibroblast activation. We found that transverse aortic constriction induced both myocardial interstitial and perivascular collagen deposition. Quantitative reverse transcription-PCR (qRT-PCR) analysis showed that the expression level of miR-19a-3p was increased in the myocardial tissues of cardiac fibrosis, and TGF-β1 induced an upregulation in miR-19a-3p expression in cardiac fibroblasts. The dual-luciferase reporter assay and qRT-PCR verified that miR-19a-3p directly bound to the 3′ untranslated regions of BAMBI mRNA, thereby reducing BAMBI expression and diminishing its ability to inhibit the TGF-β1 signaling pathway. Furthermore, overexpression of miR-19a-3p mimic increased the activation of TGF-β1/SMAD2/3 pathway signaling, promoting cardiac fibroblast activation. However, this activation was blocked by BAMBI overexpression. These findings imply that miR-19a-3p enhances the activation of TGF-β1/SMAD2/3 by inhibiting BAMBI, further boosting the activation of cardiac fibroblasts and contributing to myocardial fibrosis.