Surface plasmon resonance generates strong light localization in plasmonic nanostructures, which has been used to enhance various nonlinear optical processes. However, the nonlinear optical efficiency of metal itself is usually not high enough and this limits the potential application. Hence, if a material with a large nonlinear optical response can be embedded within the plasmonic nanostructure, it would be possible to generate larger nonlinear optical response. In this work, an organic conjugated polymer with large third harmonic generation efficiency is integrated into a gold/ITO-based plasmonic crystal. Large surface plasmon-enhanced third harmonic generation in the metal–polymer hybrid plasmonic crystal is experimentally observed. The integration of organic polymer into plasmonic crystal opens a new avenue to develop novel nonlinear optical plasmonic devices.