Elucidating elementary mechanisms that underlie bacterial diversity is central to ecology^1,2 and microbiome research³. Bacteria are known to coexist by metabolic specialization⁴, cooperation⁵ and cyclic warfare^6–8. Many species are also motile⁹, which is studied in terms of mechanism^10,11, benefit^12,13, strategy^14,15, evolution^16,17 and ecology^18,19. Indeed, bacteria often compete for nutrient patches that become available periodically or by random disturbances^2,20,21. However, the role of bacterial motility in coexistence remains unexplored experimentally. Here we show that—for mixed bacterial populations that colonize nutrient patches—either population outcompetes the other when low in relative abundance. This inversion of the competitive hierarchy is caused by active segregation and spatial exclusion within the patch: a small fast-moving population can outcompete a large fast-growing population by impeding its migration into the patch, while a small fast-growing population can outcompete a large fast-moving population by expelling it from the initial contact area. The resulting spatial segregation is lost for weak growth–migration trade-offs and a lack of virgin space, but is robust to population ratio, density and chemotactic ability, and is observed in both laboratory and wild strains. These findings show that motility differences and their trade-offs with growth are sufficient to promote diversity, and suggest previously undescribed roles for motility in niche formation and collective expulsion–containment strategies beyond individual search and survival.