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Despite the promising applications of copper selenide nanoparticles, an in-depth elucidation of the inherent properties of tetragonal Cu2Se (β-Cu2Se) has not been performed because of the lack of a facile synthesis on the nanoscale and an energy-intensive strategy is usually employed. In this work, a facile wet-chemical strategy, employing HCOOH as reducing agent, has been developed to access single-crystalline metastable β-Cu2Se hyperbranched architectures for the first time. The process avoids hazardous chemistry and high temperatures, and thus opens up a facile approach to the large-scale low-cost preparation of metastable β-Cu2Se hyperbranched architectures. A possible growth mechanism to explain the formation of the β-Cu2Se dendritic morphology has been proposed based on time-dependent shape evolution. Further investigations revealed that the metastable β-Cu2Se can convert into the thermodynamically more stable cubic α-Cu2− xSe maintaining the dendritic morphology. An increase in electrical conductivity and a tunable optical response were observed under ambient conditions. This behavior can be explained by the oxidation of the surface of the β-Cu2Se hyperbranched structures, ultimately leading to solid-state phase conversion from β-Cu2Se into superionic conductor α-Cu1.8Se, which has potential applications in energy-related devices and sensors. [ABSTRACT FROM AUTHOR]