The extracellular matrix (ECM) provides structural support to cells, thereby forming a functional tissue. In cancer, the growth of the tumor creates internal mechanical stress, which, together with the remodeling activity of tumor cells and fibroblasts, alters the ECM structure, leading to an increased stiffness of the pathological ECM. The enhanced ECM stiffness, in turn, stimulates tumor growth and activates tumor-promoting fibroblasts and tumor cell migration, leading to metastasis and increased therapy resistance. While the relationship between matrix stiffness and migration has been studied before, their connection to internal tumor stress remains unresolved. Here we used 3D ECM-embedded spheroids and hydrogel particle stress sensors to quantify and correlate internal tumor spheroid pressure, ECM stiffness, ECM remodeling, and tumor cell migration. We note that 4T1 breast cancer spheroids and SV80 fibroblast spheroids showed increased invasion—described by area, complexity, number of branches, and branch area—in a stiffer, cross-linked ECM. On the other hand, changing ECM stiffness only minimally changed the radial alignment of fibers but highly changed the amount of fibers. For both cell types, the pressure measured in spheroids gradually decreased as the distance into the ECM increased. For 4T1 spheroids, increased ECM stiffness resulted in a further reach of mechanical stress into the ECM, which, together with the invasive phenotype, was reduced by inhibition of ROCK-mediated contractility. By contrast, such correlation between ECM stiffness and stress-reach was not observed for SV80 spheroids. Our findings connect ECM stiffness with tumor invasion, ECM remodeling, and the reach of tumor-induced mechanical stress into the ECM. Such mechanical connections between tumor and ECM are expected to drive early steps in cancer metastasis.