Antioxidants and antioxidant activity confer important protective effects in plants against the effects of free radicals, which are generated by biotic and abiotic stresses, such as cold. In another study, we identified SlICE1 as a basic helix-loop-helix transcription factor to improve cold tolerance. Here, we demonstrate that SlICE1 plays an important role in the accumulation of antioxidants and in the regulation of antioxidant activity in tomato Solanum lycopersicum. Overexpression of SlICE1 in tomatoes enhanced the accumulation of antioxidants, such as beta-carotene, lycopene, and ascorbic acid, as well as antioxidant activity, measured as the scavenging of 2,2-diphenyl-1-picrylhydazyl (DPPH) free radicals and O-2(-) radicals. Furthermore, the sugar content in SlICE1-overexpressing tomatoes red fruits was higher than that in wild-type red fruits. Metabolite profiling analysis performed by capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS) revealed that several amino acids and amines were more highly accumulated in SlICE1-overexpressing tomato red fruits compared to those in wild-type tomatoes. These results suggest that SlICE1 plays a role in the regulation of antioxidant activity through the accumulation of several antioxidants.