Trichomonas vaginalis is the most widespread non-viral pathogen of the human urogenital tract, infecting ∼ 3% of the world's population annually. At the onset of infection the protist changes morphology within minutes: the flagellated free-swimming cell converts into the amoeboid-adherent stage. The molecular machinery of this process is not well studied, but is thought to involve actin reorganization. We have characterized amoeboid transition, focusing in particular on TvFim1, the only expressed protein of the fimbrin family in Trichomonas. Addition of TvFim1 to actin polymerization assays increases the speed of actin filament assembly and results in bundling of F-actin in a parallel and anti-parallel manner. Upon contact with vaginal epithelial cells, the otherwise diffuse localization of actin and TvFim1 changes dramatically. In the amoeboid TvFim1 associates with fibrous actin bundles and concentrates at protrusive structures opposing the trailing ends of the gliding amoeboid form and rapidly redistributes together with actin to form distinct clusters. Live cell imaging demonstrates that Trichomonas amoeboid stages do not just adhere to host tissue, rather they actively migrate across human epithelial cells. They do so in a concerted manner, with an average speed of 20 μm min−1 and often using their flagella and apical tip as the leading edge.