Free-form creation of 3-dimensional (3D) structures, such as in additive manufacturing (AM) and 3D printing (3DP), typically requires a direct line-of-sight or physical contact between an energy source and a build material. By stepping away from this equipment paradigm, we discovered a method to achieve 3D composites inside of opaque, open-cell foams that enables unprecedented access to bicontinuous, interlocked composite structures. We found that high-intensity focused ultrasound (HIFU) provided efficient, localized heating at a focal point that could be spatially controlled within a foam matrix. Foam specimens were infused with thermally curable acrylate resin formulations, which enabled free-form creation of 3D structures as the HIFU focal point was moved throughout the interior of the foam. The 3D structure was created entirely based upon the toolpath, without any build plate or inherently sequenced layer-by-layer processes. Since the foam and cured resin were mechanically interlocked in the process, HIFU curing achieves bicontinuous composites seemingly independent of surface compatibilities between the foam and resin. Starting with commercially available polyurethane foams, we investigated combinations with different resin systems to achieve a range of mechanical properties from the final composite structures. For example, using poly(ethylene glycol) diacrylate (PEGDA) resulted in stiff, hard composite domains within the foam, whereas resins comprising 2-hydroxyethyl acrylate (HEA) led to soft, elastomeric composite structures. Multimaterial composites were also achieved, simply by displacing uncured resin from the foam and exchanging it with a different resin formulation. Control over the shape and orientation of internal structural features within the foam scaffolds also enabled controllable anisotropic mechanical responses from the composites.