A surface plasmon resonance (SPR) sensor highly sensitive and selective for the herbicide atrazine was composed by immobilizing atrazine-imprinted polymer with gold nanoparticles on a gold thin film as a sensor chip. In the detection, the atrazine-imprinted polymer was expected to work as synthetic receptor for selectively capturing atrazine in organic solvent, and the gold nanoparticles were expected to exhibit a coupling effect with the gold thin film to enhance the local electromagnetic field between the nanoparticles and the gold film, making the sensor chip highly sensitive for changes in microenvironmental polarity. Thus, a combination of the atrazine-imprinted polymer and gold nanoparticles enabled us to compose an SPR sensor demonstrating the detection of 5 pM atrazine in acetonitrile.