Cancer metastasis is a multi-step process where cancer cells invade a distant organ. In part, the elongation of invasive protrusions mediates the initial steps of metastasis. These tumor-associated protrusions have been shown to play a key role on the invasion of individual cancer cells . However, less is known about how they regulate cancer progression on more complex multi-cellular systems, such as tumor spheroids, and how their fluctuations correlate with their metastatic potential . Herein, we analyzed the role of tumor-associated protrusion fluctuations in cancer invasion. For this, we used non-metastatic lung and more metastatic breast tumor m-spheroids encapsulated into a collagen matrix as a model. We defined novel biophysical indicators characteristic of tumor invasiveness: the frequency of probing (np) and stabilization lifetime (ts) of protrusions. We found that these morphodynamic indicators depended on the metastatic potential of tumors. In particular, non-metastatic A549 m-spheroids showed a lower np and a larger ts compared to more-metastatic MCF7 cells. The addition of doxorubicin, an anti-cancerous drug, perturbed protrusion fluctuations, increasing (decreasing) np (ts) in both tumor types. Interestingly, we identified a linear correlation between both parameters, which was in agreement with the invasive potential of the tumors. We also observed that the inhibition of the Rho pathway modulated np and ts, and abolished the invasive capacity of tumors. Finally, we used a tumor-on-a-chip model integrating our tumor spheroids and endothelial cells to investigate the crosstalk between both cell types. We found that the endothelial cells modulated both protrusion morphodynamics and their orientation. All these observations were used to define a ´predictive´ invasion index integrated into a phase diagram, which provided a novel landscape to determine the invasiveness potential of tumors based solely on their protrusion activity. Overall, the obtained results illustrate that protrusion fluctuations are key players in the mechanism of tumor invasion; therefore, they may be employed as early indicators of the metastatic potential of tumors.