It is commonly recognized that vegetables in their best season are delicious and have high nutritional value. We investigated the monthly and yearly variations in radical-scavenging (RS) activity, an indicator of antioxidant activity, ascorbic acid, and degrees Brix of Brassica rapa var. perviridis (komatsuna) cultivated in the Kanto region, Japan. RS activity was significantly higher in December, January, and February than in the other months in all tested years (2010–2015). For two varieties (summer and winter varieties) their RS activity was significantly higher in winter than in summer. The weight proportion of leaf blades and petioles at harvest was similar (leaf blade:petiole = 4:6) both in the summer and winter, indicating that the difference in the RS activity of komatsuna between the two seasons was not due to the weight ratio, but to the increase in the RS activity of leaf blades in the winter. There were no significant differences in the RS activity of komatsuna between summer (30 days) and winter (80 days) cultivation periods, greenhouse and open-field cultivations, and producers. Multiple years of field studies revealed that the antioxidant activity, ascorbic acid content, and degrees Brix of komatsuna in the Kanto region were higher in winter than in other seasons, and suggested that komatsuna with a similar quality level may be produced when grown in regions with similar weather conditions.