Due to their increased production and commercial applications, capped silver nanoparticles (AgNPs) have inevitably found their way into aquatic ecosystems. The mobility (fate/transport), bioavailability, reactivity, and toxicity of capped AgNPs are highly influenced by their colloidal stability. This study investigated the aggregation kinetics and interfacial interactions of tannic acid (TA)-coated or silica-coated AgNPs in natural organic matter (NOM)-containing electrolyte solutions by time-resolved dynamic light scattering and atomic force microscopy. Three well-characterized NOM fractions of different characteristics were selected. In Na+-solutions, the polymeric TA induced more stability to AgNPs than the hard silica coating. Although all NOM fractions weakly interacted with TA even at high Na+ concentrations, these organics adsorbed on the silica-coated AgNPs; thus, inducing stability. Humic fractions provided higher colloidal stability due to stronger electrostatic/steric interactions. Ca2+ increased the aggregation kinetics of both capped nanoparticles in the absence and presence of NOM. However, the aggregation kinetics of TA-coated AgNPs in humic NOM-containing solution were higher than those of non-humic due to a higher content of deprotonated carboxyl groups and cation bridging mechanisms. The knowledge compiled in this study would assist in understanding and predicting the fate and transport of capped nanoparticles in natural aquatic systems of different compositions.