Purple sweet potatoes are rich in the purple pigment anthocyanin. In recent years, it has been reported that the anthocyanin content of the same variety varies depending on the place of production. Therefore, to investigate the effect of soil temperature on the anthocyanin content of sweet potato tuberous roots, two types of covering materials, black and white mulch films, were used in the same field. The vines were planted in May, June and July; the cultivation period was set to 120–180 days and tuberous roots were harvested in September, October and November. The average soil temperature ranged from 22.9 to 26.5°C, with the white mulch having lower soil temperatures than the black mulch. The tuberous root yield increased with longer cultivation periods; the tuberous root yield in the May–November plot with a 180-day cultivation period was about twice that of the June–October and July–November plots with a 120-day cultivation period. The anthocyanin content of the tuberous root was negatively correlated with the average soil temperature; the test plots harvested in November had a higher anthocyanin content than the other test plots. In particular, the anthocyanin content of sweet potato cultivated in white mulch in July–November was about twice that cultivated in May–September. Although the factors that increase the tuberous root yield (prolonged cultivation period) are different from those that increase the anthocyanin content (lower temperature range), the tuberous root yield showed a larger effect on the total anthocyanin yield. Since the cultivation period needs to be prolonged to increase tuberous root yield, it would consequently increase the anthocyanin yield. The quality of the harvest was better under lower soil temperature as it led to an increase in the anthocyanin yield. Therefore, it was suggested that anthocyanin yields could be maximized by extending the growing season and harvesting at lower soil temperatures.