Cardiac fibrosis is associated with various cardiovascular diseases and can eventually lead to heart failure. Dysregulation of long non‐coding RNAs (lncRNAs) are recognized as one of the key mechanisms of cardiac diseases. However, the roles and underlying mechanisms of lncRNAs in cardiac fibrosis have not been explicitly defined. Here, we investigated the role of an antisense (AS) lncRNA from the Ras association domain‐containing protein 1 isoform A (RASSF1A) gene locus, named RASSF1‐AS1, in the development of cardiac fibrosis. Cardiac fibrosis mouse model was established by isoproterenol injection. We found that RASSF1A protein was downregulated, whereas RASSF1‐AS1 was markedly upregulated during cardiac fibrosis. Overexpression and knockdown of mouse primary cardiac fibroblasts showed that RASSF1‐AS1 negatively regulated RASSF1A expression at the post‐transcriptional level. According to the landscape analysis and sense‐AS binding evaluation, RASSF1‐AS1 partially overlaps with RASSF1A messenger RNA (mRNA) at the exon2 region. RNA pull‐down and luciferase activity assays confirmed that RASSF1‐AS1 directly bound to RASSF1A mRNA and suppressed its translation. Furthermore, wild‐type RASSF1‐AS1 had a promoting effect on nuclear factor‐κB activation and cardiac fibrosis, but mutated RASSF1‐AS1, in which the binding region was deleted, had no effect. In conclusion, RASSF1‐AS1 inhibits the translation of RASSF1A to exacerbate cardiac fibrosis in mice, indicating a potential application of RASSF1‐AS1 as a therapy target for cardiac fibrosis.