Black titanium dioxide nanoparticles (bTiO₂ NPs) have emerged as promising materials for light-driven applications due to their extended optical absorption and reduced environmental concerns compared with heavy-metal-based quantum dots. In this work, bTiO₂ NPs were synthesized via a simple, low-cost, and environmentally friendly one-pot aqueous method. X-ray diffraction analysis confirmed the formation of the anatase TiO₂ phase. Transmission electron microscopy (TEM) was used to determine the particle size finding a value of 10.3 ± 3.4 nm. The colloidal dispersion exhibited a positive zeta potential of +47.5 mV, indicating favorable electrostatic stabilization. Optical characterization revealed strong UV absorption at 280 nm, together with a pronounced NIR absorption band centered at approximately 804 nm, consistent with optical transitions associated with sub-band gap electronic states. The analysis of X-ray photoelectron spectroscopy (XPS) to the Ti 2p core-level confirmed the presence of 7.3% of Ti³⁺ species that can explain the NIR absorption. Photoluminescence analysis showed an emission band at 430–433 nm. Under 808 nm NIR irradiation, with an initial temperature of 37 °C, the bTiO₂ NPs exhibited pronounced photothermal heating, reaching a maximum temperature increase of 30.2 °C and a photothermal conversion efficiency of 21.6%. The photothermal response increased with nanoparticle concentration under the evaluated experimental conditions, demonstrating concentration-dependent NIR photothermal heating. In vitro cytotoxicity assays showed high cell viability at the lower concentration tested, exceeding 100% at 50 µg/mL, whereas cell viability decreased to approximately 60% at 500 µg/mL. Overall, these results indicate that the synthesized bTiO₂ NPs exhibit extended NIR absorption, concentration-dependent photothermal heating, and concentration-dependent effects on cell viability, supporting their potential for further investigation as photothermal nanomaterials.