AbstractIntegration of cyanine dyes and metal ions into one nanoplatform via metal‐coordination interactions is an effective strategy to build multimodality phototheranostics. The multifunctionalities of the formed nanoscale metal–organic particles (NMOPs) have been widely explored. However, the effect of metal‐coordination interaction on the aggregation behavior of cyanine dyes is rarely reported. Herein, we reported the H‐aggregation behavior of cyanine dye Cy‐3COOH induced by different metal ions M (Fe²⁺ or Mn²⁺). Moreover, the extent of H‐aggregates varied with different metal‐coordination interactions. Upon NIR irradiation, H‐aggregates of Cy‐3COOH remarkably promoted photothermal conversion efficiency. Interestingly, we also find that H‐aggregates of Cy‐3COOH induced by metal ions can generate the reactive oxygen species (ROS) involving singlet oxygen (¹O₂) and superoxide anion radical (O₂⁻⋅) upon light irradiation. In addition, the ROS efficiency varies depending on the extent of H‐aggregates. Additionally, the photoinduced ROS could disassemble aggregates and decompose cyanine dye Cy‐3COOH, which limits the photothermal capability of Cy‐3COOH/M NPs. Therefore, the photothermal performance of Cy‐3COOH/M NPs could be manipulated by the degree of H‐aggregation. This would provide a new insight to develop efficient phototheranostics NMOPs for cancer treatment.