Tumor progression is regulated by a complex interplay between neoplastic cells and the tumor microenvironment. Tumor-associated macrophages have been shown to promote breast cancer progression in advanced disease and more recently, in early stage cancers. However, little is known about the macrophage-derived factors that promote tumor progression in early stage lesions. Using a p53-null model of early stage mammary tumor progression, we found that Gas6 is highly expressed in pre-invasive lesions associated with increased infiltrating macrophages, as compared with those with few recruited macrophages. We show that F4/80+CD11b+ macrophages produce Gas6 in premalignant lesions in vivo, and that macrophage-derived Gas6 induces a tumor-like phenotype ex vivo. Using a 3-D co-culture system, we show that macrophage-derived Gas6 activates its receptor Axl and downstream survival signals including Akt and STAT3, which was accompanied by altered E-cadherin expression to induce a malignant morphology. In vivo studies demonstrated that deletion of stromal Gas6 delays early stage progression and decreases tumor formation, while tumor growth in established tumors remains unaffected. These studies suggest that macrophage-derived Gas6 is a critical regulator of the transition from premalignant to invasive cancer, and may lead to the development of unique biomarkers of neoplastic progression for patients with early stage breast cancer, including ductal carcinoma in situ.