Musashi-1 (Msi-1) is a well-established stem cell marker in both normal and malignant colon cells and it acts by positively regulating the Notch pathway through inactivation of Numb, a Notch signalling repressor. To date, the mechanisms of regulation of Msi-1 levels remain largely unknown. Here we investigated the regulation of Msi-1 by Notch signalling in the colorectal cancer cell lines MICOL-14tum and LoVo and in primary cultures of colorectal cancer (CRC) metastases. Stimulation by the Notch ligand DLL4 was associated with an increase of Msi-1 mRNA and protein levels, and this phenomenon was prevented by the addition of antibody neutralizing Notch2/3 but not Notch1. Moreover, forced expression of activated Notch3 increased Msi-1 levels, whereas silencing of Notch3 by shRNA reduced Msi-1 levels in both CRC cells and tumor xenografts. Consistent with these findings, forced Notch3 expression or stimulation by DLL4 increased levels of activated Notch1 in MICOL-14tum and LoVo cells. Finally, treatment of CRC cells with anti-Notch2/3 antibody increased Numb protein while significantly reducing formation of spheroids in both MICOL-14tum cells and primary tumor cultures. This novel feed-forward circuit involving DLL4, Notch3, Msi-1, Numb and Notch1 may be relevant for regulation of Notch signalling in physiological processes as well as in tumor development. With regard to therapeutic implications, Notch3-specific drugs could represent a valuable strategy to limit Notch signaling in the context of colorectal cancers overexpressing this receptor.