K2.1 (gene: Kcnk2), a two-pore-domain potassium channel, regulates leukocyte transmigration across the blood-brain barrier by a yet unknown mechanism. We demonstrate that Kcnk2 mouse brain microvascular endothelial cells (MBMECs) exhibit an altered cytoskeletal structure and surface morphology with increased formation of membrane protrusions. Cell adhesion molecules cluster on those protrusions and facilitate leukocyte adhesion and migration in vitro and in vivo. We observe downregulation of K2.1 and activation of actin modulating proteins (cofilin 1, Arp2/3) in inflamed wildtype MBMECs. In the mechanosensitive conformation, K2.1 shields the phospholipid PI(4,5)P from interaction with other actin regulatory proteins, especially cofilin 1. Consequently, after stimulus-related K2.1 downregulation and dislocation from PI(4,5)P, actin rearrangements are induced. Thus, K2.1-mediated regulatory processes are essential for actin dynamics, fast, reversible, and pharmacologically targetable.