A reversed-phase high-performance liquid chromatography method with diode array detection was developed for the determination of trans- and cis-resveratrol in red wines. Separation was achieved after direct injection by the use of a BDS Hypersil C18 column (250 × 4.6 mm) with gradient elution (solvent A: acetic acid 2%, solvent B: acetonitrile). Detection of trans- and cis-resveratrol was performed at 306 and 286 nm, respectively. The retention times of trans- and cis-resveratrol were 22.2 and 26.1 min, respectively. Good linearity and precision were obtained for the two isomers. Detection limits of 0.004 mg/L for trans-resveratrol and 0.02 mg/L for cis-reveratrol were obtained. The developed method was applied to determine cis- and trans-resveratrol in 30 red wines produced in Oltenia (southwestern Romania). The wines came from different vineyards harvested in various vintages. The concentration of trans-resveratrol ranged from 0.287 to 7.188 mg/L, while the content of cis-resveratrol ranged from 0.718 to 6.587 mg/L. The highest amount of trans-resveratrol was found in Merlot from Vanju Mare, Mehedinti (7.188 mg/L), followed by Sirah from Corcova, Mehedinti (4.738 mg/L), both from the 2010 harvest. The paper also approaches the study of the transformation of trans-resveratrol into the cis form after ultraviolet irradiation through glass and quartz. At the irradiation of a trans-resveratrol solution through quartz, the formation of another two compounds apart from cis-resveratrol was observed.