Micro- and nanoplastic (MNP) pollution in aquatic ecosystems requires investigation on its source, transport, and extent to assess and mitigate its risks. Chitosan is a potential biomolecule for water treatment, but its interaction with MNP is undefined. In this work, chitosan-nanoplastic interaction was explored in the laboratory under environmentally relevant conditions using polystyrene (PS) nanoplastic (NP) as model particle to identify conditions at which PS-chitosan interaction resulted in aggregation. Aggregation limits NP transport and allows separation of NP for targeted remediation. The effect of environmental conditions (pH, salinity, dissolved organic matter (DOM) content), chitosan particle size and NP surface modification on chitosan-NP interaction was studied at various chitosan doses. PS aggregated at chitosan doses as low as 0.2 % w/w, while higher doses of chitosan resulted in re-stabilization of NP in solution, restoring the particle size to its initial value. Increasing pH, DOM, or carboxyl modification of the NP surface also improved NP stability in solution. Increased salinity of the solution caused aggregation of unmodified PS independent of chitosan, but carboxyl-modified PS remained stable and aggregated at the same chitosan doses across all salinity levels. Chitosan with low molecular weight promoted PS aggregation at lower doses. Notably, zeta potential (ZP) alone did not indicate chitosan-induced PS aggregation, which occurred independently of changes in ZP. DLVO calculations based on ZP, however, still indicated attractive interaction due to charge differences, albeit with less contrast at high pH, salinity, and DOM content. Additional insights gained in the work recommend caution when using spectrophotometric methods to assess NP removal. Overall, this study demonstrates that chitosan impacts NP transport and holds potential for water remediation of NP.