AbstractResource‐use complementarity of producer species is often invoked to explain the generally positive diversity–productivity relationships. Additionally, multi‐trophic interactions that link processes across trophic levels have received increasing attention as a possible key driver. Given that both are integral to natural ecosystems, their interactive effect should be evident but has remained hidden. We address this issue by analysing diversity–productivity relationships in a simulation experiment of producer communities nested within complex food‐webs, manipulating resource‐use complementarity and multi‐trophic animal richness. We show that these two mechanisms interactively create diverse communities of complementary producer species. This shapes diversity–productivity relationships such that their joint contribution generally exceeds their individual effects. Specifically, multi‐trophic interactions in animal‐rich ecosystems facilitate producer coexistence by preventing competitive exclusion despite overlaps in resource‐use, which increases the realised complementarity. The interdependence of food‐webs and producer complementarity in creating biodiversity–productivity relationships highlights the importance to adopt a multi‐trophic perspective on biodiversity–ecosystem functioning relationships.