Significance Metamorphosis is an important biological process by which animals alter their body structures to become sexually mature adults. We discovered that tyramine signaling through the β3-octopamine receptor plays an essential role in producing the steroid hormone ecdysone, which is critical for metamorphosis. Based on our observations, we propose that monoamine signaling acts downstream of a body size checkpoint that allows metamorphosis to occur only when a critical body weight is attained during larval development and nutrients are sufficiently abundant. This work also provides a new perspective on an evolutionarily conserved monoaminergic regulation of steroid hormone production during developmental transitions such as metamorphosis. This study provides a new understanding of how metamorphosis is coordinately regulated by nutritional conditions and developmental timing.