This study aims to report the synthesis of nitrogen-doped carbon dots (N-CDs) derived from biomass, tannic acid and polyethyleneimine, which is a N-doping agent. The N-doping of CDs is found to enhance singlet oxygen (1O2) generation under visible light irradiation. Additionally, the N-CDs work as both a reducing agent and stabiliser, and hence the N-CDs can directly reduce chloroauric acid to form gold nanoparticles (Au NPs) without adding other reducers and stabilisers. The as-prepared N-CD-Au NP nanocomposite possesses 2.3 times higher photosensitising ability for producing 1O2 than N-CDs. To the best of our knowledge, this is the first report on the enhancement of the photosensitising ability of N-CDs by Au NPs. We suggest that the plasmon coupling of Au NPs with the N-CDs causes the energy transfer from the N-CDs to the ground state oxygen (3O2), thus resulting in an increased 1O2 generation.