For an organism to develop and maintain homeostasis, cell types with distinct functions must often be separated by physical boundaries. The formation and maintenance of such boundaries are commonly attributed to local mechanisms restricted to the cells lining the boundary. Here we show that, besides these local subcellular mechanisms, the formation and maintenance of tissue boundaries involves long-lived, long-ranged mechanical events. We analyzed the formation of repulsive epithelial boundaries between two epithelial monolayers, one expressing the receptor tyrosine kinase EphB2 and one expressing its ligand ephrinB1. Upon contact, both monolayers exhibited oscillatory patterns of traction forces and intercellular stresses that spanned several cell rows and tended to pull cell-matrix adhesions away from the boundary. With time, monolayers jammed and supracellular force patterns became long-lived, thereby permanently sustaining tissue segregation. Jamming was paralleled by the emergence of deformation waves that propagated away from the boundary. This phenomenon was not specific to EphB2/ephrinB1 repulsion but was also present during the formation of boundaries with an inert interface and during fusion of homotypic epithelial layers. Our findings thus unveil a global physical mechanism that sustains tissue separation independently of the biochemical and mechanical features of the local tissue boundary.