In order to survive, proliferate and metastasise, tumour cells convey specific messages to other cells in proximal and distal environments. An important aspect of tumour cell-cell communication is the secretion of exosomes; small extracellular vesicles (EVs) that originate from endosomes. Exosomes are formed from the inward budding of the endosomal membrane, creating intraluminal vesicles (ILVs) which are then released extracellularly upon endosome-plasma membrane fusion. Exosomes carry and transfer a variety of biological material to recipient cells in a target-specific manner. PI3K is one of the most frequently mutated oncogenes in breast cancer. PI3K activity is associated with increased exosome release and altered exosome content, including increased levels of tumour invasion markers. However, the mechanisms by which PI3K regulates exosome function, and its contribution to tumour progression, are poorly understood. Here we reveal by immuno-electron microscopy (EM) analysis that a known phosphoinositide-phosphatase downstream of PI3K is enriched on ILVs and increases ILV number and size in breast cancer cells. Nanoparticle tracking and transmission-EM analyses showed this phosphatase also enhances the number and size of EVs released. EV proteomics uncovered ≥200 exclusively/differentially expressed proteins that are associated with regulation of actin cytoskeleton, insulin signalling, and cell migration. Finally, transwell assays demonstrated this phosphatase imparts pro-migratory properties on breast cancer-derived EVs in vitro. Together, this data identifies a PI3K regulatory phosphatase that mediates EV formation and content, which may contribute to breast cancer progression. Future work will explore whether PI3K-dependent EVs promote metastasis in vivo, and investigate their potential use as tumour biomarkers.