Little is known about how organ growth is monitored and coordinated with the developmental timing in complex organisms. In insects, impairment of larval tissue growth delays growth and morphogenesis, revealing a coupling mechanism. We carried out a genetic screen in Drosophila to identify molecules expressed by growing tissues participating in this coupling and identified dilp8 as a gene whose silencing rescues the developmental delay induced by abnormally growing tissues. dilp8 is highly induced in conditions where growth impairment produces a developmental delay. dilp8 encodes a peptide for which expression and secretion are sufficient to delay metamorphosis without affecting tissue integrity. We propose that Dilp8 peptide is a secreted signal that coordinates the growth status of tissues with developmental timing.