Discharge from the stormwater system is as an important pathway for contaminant transport, impacting the quantity and characteristics of dissolved organic matter (DOM) in surface water, and thus the formation of disinfection byproducts (DBPs) during downstream drinking water disinfection. In this study, DOM in stormwater pipes was characterized by size-exclusion chromatography, and the formation of 27 DBPs and halogen-specific total organic halogen (TOX) following chlorination was investigated. Overall, DOM in stormwater pipes was characterized by low molecular weight compounds and microbial-derived organics. Total DBP concentrations in chlorinated stormwaters were ∼1-15 times higher than in chlorinated surface waters. DBPs formed in stormwaters were dominated by trihalomethanes and haloacetic acids. Moreover, the DBP-associated toxicity of chlorinated stormwaters was ∼1-38 times higher than in chlorinated surface waters, and mainly due to the presence of large amount of haloacetaldehydes and haloacetonitriles. Sampling during a rainfall event suggested that stormwater discharge significantly increased DBP precursors in the surface water. The high formation and estimated toxicity of DBPs in stormwater discharge indicates this is an overlooked source of DBP precursors, posing a threat to the aquatic environment and potentially drinking water quality.