In this study, the role of nanoparticles in complex aqueous matrices such as the Baltic Sea was investigated in batch-mode experiments in which titanium dioxide nanoparticles (nano-TiO2) were tested for their ability to remove heavy metals (Cr, Pb, Cu, Zn, Cd, Ni, Co) from multicomponent spiked and non-spiked Baltic Sea water. The experimental data were analyzed using different isotherms (Langmuir, Freundlich, Dubinin–Kaganer–Radushkevich (DKR)) and models (pseudo-first-order and pseudo-second-order models, the double-exponential model, and the Weber–Morris model). The equilibria and kinetic investigations showed that metal sorption to nano-TiO2 occurs in a two-step, multilayer process and that there is strong competition for sorption sites. The results of the DKR isotherm and dilution experiment indicated weak electrostatic bonds, except for Pb. The distribution coefficient values (1.8 × 103 to 2.8 × 105 ml g−1) were consistent with the good sorbent properties of nano-TiO2 and supported the use of the particles in seawater purification technologies. However, metal-enriched nano-TiO2 may also act as an effective carriers of metals to marine sediments, which could increase their availability to benthic organisms.