The implications of engineered nanomaterials (ENMs) in the environment are often investigated using pristine particles. However, there are several biogenic and geogenic materials in natural waters that interact with and modify the surface of ENMs, thereby influencing their fate and effects. Here we studied the influence of soluble extracellular polymeric substances (sEPS) produced by freshwater and marine algae on the surface properties and fate of three commercial TiO2 nanoparticles (nTiO2) with different coatings. Adsorption of sEPS by the various nTiO2 is dependent on particle surface area, intrinsic nTiO2 surface charge, and hydrophobicity. Interactions between sEPS and nTiO2 were driven by electrostatic interactions and chemical bonding (bridge-coordination) between the COO- group of sEPS and nTiO2. Charge reversal of positively charged nTiO2 was observed at pH 7 in the presence of 0.5 mg-C/L sEPS. In addition, the critical coagulation concentration (CCC) of nTiO2 increased in the presence of sEPS-from both freshwater and marine sources. CCC of all nTiO2 increased as sEPS concentrations increased. This study shows that naturally occurring sEPS can modify the surface properties and fate of nTiO2 in natural waters, and should be accounted for when predicting the fate and effects of engineered nanomaterials in the environment.