Spheroids are multicellular systems with an interesting rheology giving rise to elasto–visco–plastic properties. They are good tumor models, but the role of the extracellular matrix (ECM) is not fully understood. ECM is an important link between cells and may have a significant impact on tissue organization. Here we determine viscoelastic properties of spheroids including different collagen I amounts using AFM and predict new frequency–dependent properties leading to soft glassy rheology behavior. A unified model – similar to single cell behavior – is proposed and discussed, while complementary confocal experiments reveal the microstructure of spheroids, with collagen I fibers serving as a skeleton for cells, thus reinforcing the spheroid viscoelastic behavior.