Metastasis is responsible for most cancer-related deaths. However, only a fraction of circulating cancer cells succeed in forming secondary tumours, indicating that adaptive mechanisms during circulation play a part in dissemination. Here, we report that constriction during microcapillary transit triggers reprogramming of melanoma cells to a tumorigenic cancer stem cell-like state. Using a microfluidic device mimicking physiological flow rates and gradual capillary narrowing, we show that compression through narrow channels causes cell and nuclear deformation, rapid chromatin remodelling and increased calcium signalling via mechanosensor PIEZO1. Within minutes, cells upregulate transcripts associated with metabolic reprogramming and metastatic processes. Over time, this results in the stable adoption of a cancer stem cell-like state. Squeezed cells express elevated melanoma stem cell markers, exhibit increased trans-endothelium invasion and display enhanced tumorigenicity in vitro and in vivo. Pharmacological inhibition of PIEZO1 blocks this transition, while activation with Yoda1 induces the stem cell-like state irrespective of constriction. Deletion of PIEZO1 completely abolishes the constriction-induced phenotype. Together, these findings demonstrate that compressive forces during circulation reprogram circulating cancer cells into tumorigenic, stem cell-like states, primed for extravasation and metastatic colonization.