The border zone (BZ) of the infarcted heart is a geographically complex and biologically enigmatic interface separating poorly perfused infarct zones (IZs) from remote zones (RZs). The cellular and molecular mechanisms of myocardial BZs are not well understood because microdissection inevitably combines them with uncontrolled amounts of RZs and IZs. Here, we use single-cell/nucleus RNA sequencing, spatial transcriptomics and multiplexed RNA fluorescence in situ hybridization to redefine the BZ based on cardiomyocyte transcriptomes. BZ1 ( ) forms a hundreds-of-micrometer-thick layer of morphologically intact cells adjacent to RZs that are detectable within an hour of injury. Meanwhile, BZ2 ( ) forms a near-single-cell-thick layer of morphologically distorted cardiomyocytes at the IZ edge that colocalize with matricellular protein-expressing myofibroblasts and express predominantly mechanotransduction genes. Surprisingly, mechanical injury alone is sufficient to induce BZ genes. We propose a 'loss of neighbor' hypothesis to explain how ischemic cell death mechanically destabilizes the BZ to induce its transcriptional response.