Top-down trophic interactions are major drivers of microbiome dynamics, yet their outcomes are difficult to predict and their consequences for pathogen control remain unclear. We combine synthetic bacterial communities of varying complexity with field studies and microcosm assays to test whether microbivorous nematodes reorganize microbiomes to suppress soilborne disease. Field studies show stronger nematode-microbe associations around healthy plants, and microcosm assays confirm that nematode presence produces stable suppression, whereas microbe-only communities collapse under pathogen invasion. Nematode predation depletes non-preferred bacterial taxa and enriches metabolically versatile taxa within Proteobacteria, increasing community-level antagonistic potential and promoting complementary resource-use interactions linked to pathogen inhibition, yielding suppression beyond individual or pairwise effects. A minimal four-component feedback loop linking a nematode predator, plant pathogens, and two plant-associated bacteria with complementary functions accounts for the emergent outcome. Together, these results reveal an animal-mediated pathway of microbiome assembly that enhances resistance to pathogen invasion and provide a trophically informed framework for designing stable, disease-suppressive microbiomes in agriculture.