The T-box family of transcription factors, defined by a conserved DNA binding domain called the T-box, regulate various aspects of embryogenesis by activating and/or repressing downstream genes. In spite of the biological significance of the T-box proteins, how they regulate transcription remains to be elucidated. Here we show that the Groucho/TLE-associated protein Ripply converts T-box proteins from activators to repressors. In cultured cells, zebrafish Ripply1, an essential component in somite segmentation, and its structural relatives, Ripply2 and -3, suppress the transcriptional activation mediated by the T-box protein Tbx24, which is coexpressed with ripply1 during segmentation. Ripply1 associates with Tbx24 and converts it to a repressor. Ripply1 also antagonizes the transcriptional activation of another T-box protein, No tail (Ntl), the zebrafish ortholog of Brachyury. Furthermore, injection of a high dosage of ripply1 mRNA into zebrafish eggs causes defective development of the posterior trunk, similar to the phenotype observed in homozygous mutants of ntl. A mutant form of Ripply1 defective in association with Tbx24 also lacks activity in zebrafish embryos. These results indicate that the intrinsic transcriptional property of T-box proteins is controlled by Ripply family proteins, which act as specific adaptors that recruit the global corepressor Groucho/TLE to T-box proteins.