The reactivity of the common pain reliever acetaminophen (paracetamol or 4-acetamidophenol) with monochloramine in aqueous solution was investigated in this study. Initially, monochloramine demands of acetaminophen (AAP) and its chlorination byproduct, 1,4-benzoquinone, were determined in excess of oxidant at pH 8.5 and 25 °C. Monochloramine demands of AAP and 1,4-benzoquinone were respectively 1.2 (±0.2) and 4.4 (±0.3) mol/mol after 72 h of reaction. Chlorination reactions were also conducted under the same experimental conditions in order to compare the oxidant demands and the reactivity of free chlorine and monochloramine toward AAP and 1,4-benzoquinone. Chlorine demands of AAP and 1,4-benzoquinone were much more important than monochloramine demands and respectively accounted for 13.4 (±0.2) and 6.8 (±0.5) mol/mol. The reaction of AAP with monochloramine was monitored using reversed-phase high performance liquid chromatography with ultraviolet absorbance (HPLC-UV). AAP reacted with monochloramine to form simultaneously two first byproducts. The half-life of reaction was estimated to be 90 min. The apparent second-order rate constant (kapp2) for the reaction of AAP with monochloramine at pH 8.5 was 231 M−1·h−1.