Introduction: The marine luminescent bacterium Aliivibrio fischeri is currently used to assess the toxicity of environmental contaminants. Heavy metal ions such as Co²⁺, Zn²⁺, and Cu²⁺ in aquatic samples are known to inhibit bacterial bioluminescence at high concentrations. These metal ions form Werner-type complexes upon reaction with bidentate ligands. However, the changes in bioluminescence inhibition associated with the stepwise formation of metal complexes have not been fully elucidated. Hormesis—a biphasic dose–response relationship in which low doses induce stimulation and high doses cause inhibition—may contribute to this phenomenon. This study aimed to investigate the hormetic effects of metal complex formation on the bioluminescence intensity of Aliivibrio fischeri. Methods: Octahedral complexes of Co²⁺, Zn²⁺, and Cu²⁺ were prepared by mixing the metal ions with varying concentrations of ethylenediamine. The bioluminescence intensity of Aliivibrio fischeri was measured using the ROTAS™ Leachable kit. The redox potential (ORP) of each complex solution was also measured, as ORP is known to influence the bacterial respiratory chain. The degree of hormetic response was evaluated by calculating the hormetic area from time–course data of bioluminescence intensity. Results: All metal ions exhibited hormetic responses upon complexation with ethylenediamine. The hormetic area increased as ORP decreased in the sample solutions (Co²⁺: r = 0.89, Zn²⁺: r = 0.96, Cu²⁺: r = 0.73). The relatively weak correlation in Cu²⁺ complexes is likely attributable to the Jahn–Teller effect characteristic of Cu²⁺. Conclusions: Heavy metal ions tend to exhibit hormesis via the formation of Werner-type complexes, and this behavior is influenced by ORP, which in turn depends on the structure of the metal complexes.