Hydroxyapatite (HAp)/carbon nanotubes (CNTs) hybrid composite materials are successfully synthesized via a biomineralization process that employs poly(dopamine) (PDA), a synthetic mimic of mussel adhesive proteins. Creating bio-inorganic composites for regenerative medicine requires appropriate fillers to enhance their mechanical robustness; for example, natural bones are composed mainly of HAp supported by collagen fibers. In this regard, many efforts have been made to harness HAp as a bone substitute through its integration with reinforcing fibrous materials such as CNTs. We found that the formation of a PDA ad-layer on the surface of CNTs changed the hydrophobic CNTs to become bioactive. This enabled efficient interaction between the CNTs and mineral ions (e.g., Ca2+), which facilitated the mineralization of HAp. CNTs functionalized with PDA (CNT-PDA) highly accelerated the formation of HAp when incubated in a simulated body fluid and exhibited a minimal cytotoxic effect on bone osteoblast cells compared to pristine or carboxylated CNTs. Our results show the potential of CNT-PDA as a scaffold material for bone tissue regeneration and implantation.