Steroid hormones regulate tissue development and physiology by modulating the transcription of a broad spectrum of genes. In insects, the principal steroid hormones, ecdysteroids, trigger the expression of thousands of genes through a cascade of transcription factors (TFs) to coordinate developmental transitions such as larval molting and metamorphosis. However, whether ecdysteroid signaling can bypass transcriptional hierarchies to exert its function in individual developmental processes is unclear. Here, we report that a single non-TF effector gene mediates the transcriptional output of ecdysteroid signaling in Drosophila myoblast fusion, a critical step in muscle development and differentiation. Specifically, we show that the 20-hydroxyecdysone (commonly referred to as "ecdysone") secreted from an extraembryonic tissue, amnioserosa, acts on embryonic muscle cells to directly activate the expression of antisocial (ants), which encodes an essential scaffold protein enriched at the fusogenic synapse. Not only is ants transcription directly regulated by the heterodimeric ecdysone receptor complex composed of ecdysone receptor (EcR) and ultraspiracle (USP) via ecdysone-response elements but also more strikingly, expression of ants alone is sufficient to rescue the myoblast fusion defect in ecdysone signaling-deficient mutants. We further show that EcR/USP and a muscle-specific TF Twist synergistically activate ants expression in vitro and in vivo. Taken together, our study provides the first example of a steroid hormone directly activating the expression of a single key non-TF effector gene to regulate a developmental process via inter-organ signaling and provides a new paradigm for understanding steroid hormone signaling in other developmental and physiological processes.