We report a method for determining dopamine in serum via colorimetric sensing. This new method adopts bromelain-templated MnO₂ nanosheets that possess high oxidase activity. 3,3′,5,5′-Tetramethylbenzidine (TMB) can be directly oxidized by bromelain-MnO₂ nanosheets to oxTMB products. Without dopamine, the absorbance at 652 nm (A₆₅₂) of oxTMB products is significantly enhanced. Upon the addition of dopamine, oxidase activity is markedly inhibited because the lamellar structure decomposes to generate Mn²⁺ ions, and the A₆₅₂ value gradually decreases. Moreover, the oxidation process of dopamine can be accelerated by Mn²⁺ ions, and the two hydroxyl groups on the annulated benzene of dopamine can be oxidized to form a light pink benzoquinone compound. Therefore, an economical, visual colorimetric sensing method was developed for the determination of dopamine. When the concentration of dopamine is between 100 nM and 10.0 μM, there is a linear relationship between the A₆₅₂ value and the dopamine concentration. The limit of detection (LOD) was 39.8 nM. The colorimetric sensing was successfully utilized in blank spike experiments with human serum samples and fetal bovine serum samples, and the dopamine content in human serum samples could be quantitatively and accurately determined.