Hizikia fusiforme is a medicinal and edible brown alga, while the reparative effects and mechanisms of its fermentation products against ultraviolet B (UVB)-induced skin inflammation and barrier damage have rarely been reported. To investigate the reparative effects, Hizikia fusiforme fermentation broth was prepared by fermentation with Lactiplantibacillus plantarum, and biochemical analyses, cellular experiments, and erythrocyte hemolysis assays were performed to evaluate its bioactive substance content, antioxidant activity, anti-inflammatory activity, barrier-repairing effects, and safety. The bioactive substance content and in vitro antioxidant capacity of the fermentation broth were found to be higher than those of the aqueous extract. After treatment with 5% fermentation broth, the viability of UVB-damaged cells was increased by 18.92% compared with that of the model group, the level of reactive oxygen species (ROS) was decreased by 52.16%, and the protein levels of inflammatory factors were decreased by 24.37%–72.98%. In addition, the abnormal expression of aquaporin-3 (AQP3), filaggrin (FLG), and caspase-14 (Caspase-14) was ameliorated. The transcription levels of genes related to the mitogen-activated protein kinase (MAPK)/nuclear factor kappa-B (NF-κB) signaling pathway were decreased by 18.77%–32.14% compared with those in the model group, and a low erythrocyte hemolysis rate was observed for the fermentation broth. In conclusion, Hizikia fusiforme fermentation broth exhibited strong in vitro antioxidant activity. UVB-induced oxidative stress in HaCaT cells was alleviated, and the abnormal expression of inflammatory cytokines and barrier-related factors was ameliorated following treatment with the fermentation broth. These findings provide experimental evidence for its further investigation and development.