MUC1 is a transmembrane glycoprotein abnormally expressed in human adenocarcinomas. The extracellular domain of MUC1 contains a variable number of tandem repeats (VNTR) region that is extensively O-glycosylated in normal epithelia and underglycosylated in tumor cells. This change in posttranslational modification of MUC1 leads to changes in its normal functions including, we hypothesized, its interaction with other molecules. We identified CIN85, an adaptor protein involved in multiple cellular processes including signal transduction, cytoskeletal remodeling and cancer cell invasion, as one of several proteins that associate with MUC1 in tumor cells. CIN85 associates with both the cytosolic tail and the extracellular VNTR of MUC1. Co-immunoprecipitation and confocal immunofluorescence confirmed that MUC1 and CIN85 co-localize primarily at the plasma membrane but the complex can be found also in the cytosol and on the cytoskeleton. MUC1 and CIN85 are both over-expressed in early as well as advanced clinical stages of breast cancer and co-localize on invadopodia-like structures implicated in cell invasion. siRNA-mediated silencing of CIN85 and/or MUC1 revealed that MUC1 enhances CIN85-dependent breast cancer cell migration and invasion in vitro. However, ectopic expression of MUC1 enhances the motility induced by CIN85. When tested in vivo in a tumor metastasis model of B16 melanoma, mice injected with CIN85-depleted melanoma cells exhibited few or no lung metastasis and, similarly to the in vitro results, overexpression of MUC1 recovered the shCIN85-reduced metastatic process. Our findings implicate this newly identified CIN85/MUC1 complex associated with invadopodia-related molecules in promoting the invasive and metastatic potential of breast cancer.