Soot nanoparticles (SNPs) produced from incomplete combustion have strong impacts on aquatic environments as they eventually reach surface water, where their environmental fate and transport are largely controlled by aggregation. This study investigated the aggregation kinetics of SNPs in the presence of macromolecules including fulvic acid (FA), humic acid (HA), alginate polysaccharide, and bovine serum albumin (BSA, protein) under various environmentally relevant solution conditions. Our results showed that increasing salt concentrations induced SNP aggregation by suppressing electrostatic repulsion and that CaCl2 exhibited stronger effect than NaCl in charge neutralization, which is in agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation rates of SNPs were variously reduced by macromolecules, and such stabilization effect was the greatest by BSA, followed by HA, alginate, and FA. Steric repulsion resulting from macromolecules adsorbed on SNP surfaces was mainly responsible for enhancing SNP stability. Such steric repulsion appeared to be affected by macromolecular structure, as BSA having a more compact globular structure on SNP surfaces imparted long-range steric repulsive forces and retarded the SNP aggregation rate by 10–100 times. In addition, alginate was shown to enhance SNP aggregation by ∼10 times at high CaCl2 concentrations due to alginate gel formation via calcium bridging. The results may bear strong significance for the fate and transport of SNPs in both natural and controlled environmental systems. Display omitted •Stabilization of SNPs by macromolecules follows the order of BSA > HA > alginate > FA.•SNP stabilization occurs via macromolecule adsorption that imparts steric repulsion.•Adsorption affinity and structure of macromolecules affect stabilization of SNPs.•BSA adsorbed on SNPs retards aggregation by 10–100 times due to long-range repulsion.•Alginate gel formed by calcium bridging with Ca2+ enhances aggregation by ∼10 times. Aggregation kinetics of SNPs in aquatic environments are significantly affected by environmental and biological macromolecules including FA, HA, alginate, and BSA.