Elevated serum uric acid (SUA) level is known to be a prognostic factor in patients with acute coronary syndrome (ACS). However, the correlation between SUA level and coronary plaque instability has not been fully evaluated. The aim of this study was to investigate the association between SUA level and plaque instability of nonculprit lesions in patients with ACS using optical coherence tomography. A total of 150 patients with ACS who underwent 3-vessel optical coherence tomography were selected. Patients were classified into 3 groups according to tertiles of SUA level. There was a trend towards a thinner fibrous cap (0.15 ± 0.06 versus 0.07 ± 0.01 versus 0.04 ± 0.01 mm2, p<0.001) and a wider mean lipid arc (169.41 ± 33.16 versus 177.22 ± 37.76 versus 222.43 ± 47.65°, p<0.001) with increasing SUA tertile. The plaques of the high and intermediate tertile groups had a smaller minimum lumen area than the low tertile group (6.02 ± 1.11 versus 5.38 ± 1.28 mm2, p<0.001). In addition, thin-cap fibroatheromas, microvessels, macrophages, and cholesterol crystals were more frequent in the high tertile group than the low and intermediate groups. Multivariate analysis showed SUA level to be a predictor of plaque instability.