The total polyphenol content and antioxidant activity were compared in the leaves of seven red amaranth (Amaranthus tricolor L.) cultivars. The effect of the sunlight level on the accumulation of total polyphenol content and antioxidant activity was also investigated by growing plants under full sunlight and shaded conditions. The total phenolic content and antioxidant activity differed among the cultivars studied, and leaves from the cultivar ‘Rocto Joba’ and ‘Rocto Lal’ had the highest phenolics and antioxidant activity, respectively. These values were lowest in the leaves of the cultivar ‘Red Queen’. Comparatively, red-fleshed cultivars had more total polyphenols and antioxidant activity than red green-fleshed cultivars. Total polyphenol and antioxidant activity were greater in leaves from plants grown under full sunlight without shading. The positive correlation between antioxidant activity and total polyphenols (R² = 0.824) suggests that phenolic compounds are the major antioxidant components in red amaranth. The results indicate that red amaranth containing high phenolics may provide a source of dietary antioxidants. The combination of cultivar variation and responsiveness to specific growing conditions can create opportunities for the production and processing of vegetable red amaranth with improved antioxidant properties.