Plexins encode receptors for semaphorins, molecular signals guiding cell migration, and axon pathfinding. The mechanisms mediating plexin function are poorly understood. Plexin activation in adhering cells rapidly leads to retraction of cellular processes and cell rounding "cell collapse"). Here we show that, unexpectedly, this response does not require the activity of Rho-dependent kinase (ROCK) nor the contraction of F-actin cables. Interestingly, integrin-based focal adhesive structures are disassembled within minutes upon plexin activation; this is followed by actin depolymerization and, eventually, by cellular collapse. We also show that plexin activation hinders cell attachment to adhesive substrates, blocks the extension of lamellipodia, and thereby inhibits cell migration. We conclude that plexin signaling uncouples cell substrate-adhesion from cytoskeletal dynamics required for cell migration and axon extension.