Cancer cachexia is one of the most common causes of death among cancer patients, no effective anti-cachectic treatment is currently available. In experimental cachectic animal models, aberrant activation of STAT3 in skeletal muscle has been found to contribute to muscle wasting. However, its clinical association, the factors regulating STAT3 activation, and the molecular mechanisms remain incompletely understood. Here, we show that an enhanced interaction between activated STAT3 and HSP90, which were observed in the skeletal muscle of cancer cachexia patients, is a crucial event for the development of cachectic muscle wasting. Administration of HSP90 inhibitors 17DMAG and PU-H71 alleviated the muscle wasting in C26 tumor-bearing cachectic mice or the myotube atrophy of C2C12 cells induced by C26 conditional medium. A mechanistic study indicated that in cachectic skeletal muscle, prolonged STAT3 activation transactivated FOXO1 by binding directly to its promoter and triggered the muscle wasting in a FOXO1-dependent manner; Our results demonstrate that the HSP90/STAT3/FOXO1 axis plays a critical role in the cachectic muscle wasting, which might serve as potential therapeutic target for the treatment of cancer cachexia.