Here, we report a simple, sensitive and selective colorimetric method for detection of dopamine (DA) using gold nanoparticles (AuNPs). AuNPs of 15 nm size were synthesized by the citrate reduction method. The method is based on the principle that the aggregated AuNPs induced by melamine can be separated by the melamine–DA conjugate through hydrogen-bonding interactions, and thus, the blue color of AuNPs changes to blue-purple. Under optimized conditions, melamine–AuNPs are highly specific for DA and can detect melamine down to a concentration of 33 nM. Furthermore, we demonstrate the application of the approach in human urine samples, which suggests its great potential for diagnostic purposes.