Global degradation of coral reefs and macroalgal beds can have ecosystem-wide implications for biodiversity, ecological functioning, and ocean resources. However, recent studies in warm temperate zones have documented community shifts from macroalgae to corals, signaling a potential mechanism for coral conservation under climate warming. Here, we present evidence that warming, aided by the dominant poleward-flowing current system, is facilitating the expansion of tropical corals and herbivorous fishes into existing temperate Japanese macroalgae communities, which are contracting faster than they are expanding. Furthermore, our results suggest future climate change may exacerbate this process, potentially compromising the long-term stability of these communities. Future conservation of these communities might require of a more proactive management toward climate adaptation.