Nonylphenol (NP) is widely distributed in the environment as a microbial metabolite of nonylphenol polyethoxylates, which are commercially important surfactants. Although there are concerns regarding the estrogenic activity of NP and its effects on wildlife, this paper reports a new detrimental effect for NP; oxidative DNA damage. After UV or sunlight irradiation, several NP photoproducts were detected by high performance liquid chromatography (HPLC). Two HPLC fractions induced formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) in calf thymus DNA. Based on gas chromatography/mass spectrometry and UV/visible spectrometry, one fraction was confirmed to contain 4-nonylcatechol. The other active compound remains unidentified. Non-irradiated NP did not exhibit any detectable 8-oxodG formation. These results suggest that NP induces oxidative DNA damage following environmental light exposure. This DNA damaging activity is a new adverse effect of NP and, in addition to estrogenic activity, should be investigated further and taken into consideration during NP risk assessment.