This study aimed to investigate the effects of different relative humidities (%) on the microbial safety, antioxidant activity, ascorbic acid, fucoxanthin, and tocopherol contents ofUndaria pinnatifidasporophyll powder (UPSP) stored for 4 weeks. The caking phenomenon did not occur in the 11–53% relative humidity conditions, but it did in the 69%, 81%, and 93% relative humidity conditions with caking index values of 88.30%, 99.75%, and 99.98%, respectively. The aerobic bacterial contents increased drastically in samples stored at 69–93% relative humidity. Ascorbic acid was unstable at high relative humidity, but fucoxanthin and tocopherol were more unstable at low relative humidity. Therefore, it was most stable at intermediate relative humidity. The 69% relative humidity sample had higher DPPH (12.57 g BHAE/kg), ABTS (4.87 g AAE/kg), and FRAP (4.60 g Fe (II)/kg) than the other samples. This study could be helpful for the storage and transport of UPSP under optimum relative humidity conditions, which can significantly prevent quality losses.