The environmental bacterium Legionella pneumophila causes a severe pneumonia termed Legionnaires' disease. L. pneumophila employs a conserved mechanism to replicate within a specific vacuole in macrophages or protozoa such as the social soil amoeba Dictyostelium discoideum. Pathogen–host interactions depend on the Icm/Dot type IV secretion system (T4SS), which translocates approximately 300 different effector proteins into host cells. Here we analyse the effects of L. pneumophila on migration and chemotaxis of amoebae, macrophages or polymorphonuclear neutrophils (PMN). Using under-agarose assays, L. pneumophila inhibited in a dose- and T4SS-dependent manner the migration of D. discoideum towards folate as well as starvation-induced aggregation of the social amoebae. Similarly, L. pneumophila impaired migration of murine RAW 264.7 macrophages towards the cytokines CCL5 and TNFα, or of primary human PMN towards the peptide fMLP respectively. L. pneumophila lacking the T4SS-translocated activator of the small eukaryotic GTPase Ran, Lpg1976/LegG1, hyper-inhibited the migration of D. discoideum, macrophages or PMN. The phenotype was reverted by plasmid-encoded LegG1 to an extent observed for mutant bacteria lacking a functional Icm/Dot T4SS.Similarly, LegG1 promoted random migration of L. pneumophila-infected macrophages and A549 epithelial cells in a Ran-dependent manner, or upon ‘microbial microinjection’ into HeLa cells by a Yersinia strain lacking endogenous effectors. Single-cell tracking and real-time analysis of L. pneumophila-infected phagocytes revealed that the velocity and directionality of the cells were decreased, and cell motility as well as microtubule dynamics was impaired. Taken together, these findings indicate that the L. pneumophila Ran activator LegG1 and consequent microtubule polymerization are implicated in Icm/Dot-dependent inhibition of phagocyte migration.