Surface-enhanced Raman spectroscopy (SERS) is an important tool for the analytical, trace detection of many inorganic and organic materials, especially for materials involved in medical care, food safety and environmental pollution. Numerous efforts have been dedicated to exploring periodic metallic materials with a high density of hotspots. However, for most periodic metallic materials fabricated by top-down and bottom-up approaches, the distribution of hotspots is restricted to one or two dimensions. Here, for the first time, we report the successful fabrication of a bio-inspired bicontinuous gyroid-structured Au SERS substrate with a high density of three-dimensionally (3D) distributed hotspots. The as-required gyroid-structured substrates were demonstrated to be highly sensitive, reproducible and uniform, with an enhancement factor of up to 10⁹. Finite-difference time domain (FDTD) simulations were conducted to reveal the mechanism leading to the high enhancement and we found that the interconnected helices in the gyroid structure not only increase the hotspot density but also contribute to increasing the scattering cross-section of the incident laser. The substrate was then adopted for the SERS detection of bis(2-ethylhexyl) phthalate, the most frequently used plasticizer in food, paints, house-hold items, perfumes and so on, and reached a detection limit of 1 fM, which is among the best results ever reported. Moreover, the mechanism deduced here will provide insight into the future design and selection of novel surface plasmonic resonance substrates, as many other bicontinuous interconnected systems are available.