Deregulated alternative splicing of the endocytic adaptor NUMB resulting in high expression of Exon9in (exon 9-containing) isoforms has been reported in several cancer types. However, the role of Numb isoform expression in tumor progression and the underlying mechanisms remain elusive. Here, we report greater exon 9 inclusion in multiple cancer types including all subtypes of breast cancer, and correlation of higher exon 9 inclusion in patients with worse prognosis. Deletion of Exon9in in breast cancer cells leads to reduced cell growth and a significant decrease of lung metastasis in orthotopic xenograft experiments. Quantitative mass spectrometry revealed downregulation of proteins involved in EMT and ECM organization and remodeling of the endocytic protein network in cells lacking the Exon9in Numb isoforms. Exon 9 deletion also results in reduced surface levels of ITGβ5, and downstream signaling to ERK and SRC, consistent with enhance lysosomal targeting mediated by the remaining Exon9sk (exon 9 skipping) Numb isoforms. Our findings reveal that Exon9in isoforms promote breast cancer progression by relieving Numb mediated down regulation of integrins and implicate Numb alternative splicing as a progression factor in multiple cancer types.