Actin filament remodeling regulates several endothelial cell (EC) processes such as contraction, migration, adhesion, and shape determination. Mitogen-activated protein kinase (MAPK)–activated protein kinase 2 (MK2)–mediated phosphorylation of heat-shock protein 27 kDa (HSP27) promotes actin filament remodeling, but little is known about the regulation of this event in ECs. We found that tumor necrosis factor-α (TNF-α) SUMOylated MK2 at lysine (K)-339 affected EC actin filament organization and migration. Loss of the MK2 SUMOylation site (MK2-K339R) increased MK2 kinase activity and prolonged HSP27 phosphorylation, enhancing its effects on actin filament-dependent events. Both TNF-α–mediated EC elongation and steady laminar shear stress–mediated EC alignment were increased by MK2-K339R. Moreover, kinase-dead dominant-negative MK2 (DN-MK2) inhibited these effects. Cell migration is a dynamic process regulated by actin filament remodeling. Both wild-type MK2 (WT-MK2) and DN-MK2 significantly enhanced TNF-mediated inhibition of EC migration, and MK2-K339R further augmented this effect. Interestingly, the p160-Rho–associated coiled-coil kinase (ROCK) inhibitor Y-27632 reversed this effect by MK2-K339R, which strongly suggests that both excessive and insufficient levels of actin filament remodeling can block EC migration. Our study shows that MK2 SUMOylation is a new mechanism for regulating actin filament dynamics in ECs.