We suggest an easy one-step underwater plasma method to generate nanoporous N-doped TiO₂(N-TiO₂) photocatalysts with high reactivity in the visible light range. The extreme energy input of this underwater plasma technology allowed the synthesis, crystallization, doping, and porosity of TiO₂ by inducing the continuous reaction of highly energetic atomic and molecular species within a few minutes. The produced N-TiO₂ demonstrated a nanoporous anatase-brookite polycrystalline structure with substitutionally doped N atoms. The N doping species led to a narrow bandgap, effective charge-carrier separation, and better light absorbing ability in the visible light region. Accordingly, N-TiO₂ photocatalysts showed efficient photocatalytic activities for methylene blue(MB), rhodamine B(Rh B), and tetracycline(TC) under solar and visible light conditions, up to approximately 4.5 times higher than that of commercial TiO₂. Furthermore, the superior biocompatibility (94.5%) of water purified by N-TiO₂ photocatalysts proved the feasibility of these materials for practical use. Moreover, N-TiO₂ photocatalysts showed excellent antibacterial ability against gram-negative Escherichia coli(E. coli) and gram-positive Staphylococcus aureus(S. aureus), which extends the applicability of N-TiO₂ photocatalysts to biomedical applications and sludge purification. This study presents one of the most facile and fast methods for the production of highly visible-light-responsive and nanoporous TiO₂ for environmental and biomedical applications.