Epithelial-to-mesenchymal transition (EMT) is crucial for tumor progression, being linked to alterations in the extracellular matrix (ECM). Understanding the ECM's role in EMT can uncover new therapeutic targets, yet replicating these interactions in vitro remains challenging. It is shown that hybrid hydrogels of alginate (ALG) and cell-derived decellularized ECM (dECM), with independently tunable composition and stiffness, are useful 3D-models to explore the impact of the breast tumor matrix on EMT. Soft RGD-ALG hydrogels (200 Pa), used as neutral bulk material, supported mammary epithelial cells morphogenesis without spontaneous EMT, allowing to define the gene, protein, and biochemical profiles of cells at different TGFβ1-induced EMT states. To mimic the breast tumor composition, dECM from TGFβ1-activated fibroblasts (adECM) are generated, which shows upregulation of tumor-associated proteins compared to ndECM from normal fibroblasts. Using hybrid adECM-ALG hydrogels, it is shown that the presence of adECM induces partial EMT in normal epithelial cells, and amplifes TGF-β1 effects compared to ALG and ndECM-ALG. Increasing the hydrogel stiffness to tumor-like levels (2.5 kPa) have a synergistic effect, promoting a more evident EMT. These findings shed light on the complex interplay between matrix composition and stiffness in EMT, underscoring the utility of dECM-ALG hydrogels as a valuable in vitro platform for cancer research.