Stormwater runoff collects various contaminants as it flows into surface water bodies. Nutrients such as phosphates and nitrates carried over by stormwater runoff significantly impact the water quality of many surface water bodies. The current study focuses on removing phosphate from stormwater runoff using batch test experiments with different adsorbent materials. Biochar (a carbon-rich material), chitosan (obtained from deacetylation of naturally abundant biopolymer chitin), and a composite material synthesized from biochar and chitosan were tested. Specifically, biochar produced from cornstalk and chitosan derived from crustacean shells were selected for the study. The batch tests were conducted at typical phosphate concentrations observed in the stormwater runoffs (1–8 mg/L) over 24 h. The results show significant adsorption capacities of 41.49 mg/kg for chitosan and 32.26 mg/kg for biochar. Additionally, the chitosan–biochar composite exhibited an enhanced phosphate removal capacity of 72.99 mg/kg, representing a 126% increase compared to biochar. The adsorption capacities were modeled using Freundlich, Langmuir, and Temkin isotherms to identify the potential mechanisms involved. The results of the study provide insight into the effectiveness of the chitosan–biochar composite for enhanced removal of phosphate from stormwater runoff.