AbstractSkeletal muscle atrophy is a common clinical feature of many acute and chronic conditions. Circular RNAs (circRNAs) are covalently closed RNA transcripts that are involved in various physiological and pathological processes, but their role in muscle atrophy remains unknown. Global circRNA expression profiling indicated that circRNAs are involved in the pathophysiological processes of muscle atrophy. circTmeff1 is identified as a potential circRNA candidate that influences muscle atrophy. It is further identified that circTmeff1 is highly expressed in multiple types of muscle atrophy in vivo and in vitro. Moreover, the overexpression of circTmeff1 triggers muscle atrophy in vitro and in vivo, while the knockdown of circTmeff1 expression rescues muscle atrophy in vitro and in vivo. In particular, the knockdown of circTmeff1 expression partially rescues muscle mass in mice during established atrophic settings. Mechanistically, circTmeff1 directly interacts with TAR DNA‐binding protein 43 (TDP‐43) and promotes aggregation of TDP‐43 in mitochondria, which triggers the release of mitochondrial DNA (mtDNA) into cytosol and activation of the cyclic GMP‐AMP synthase (cGAS)/ stimulator of interferon genes (STING) pathway. Unexpectedly, TMEFF1‐339aa is identified as a novel protein encoded by circTmeff1 that mediates its pro‐atrophic effects. Collectively, the inhibition of circTmeff1 represents a novel therapeutic approach for multiple types of skeletal muscle atrophy.