Skin is the outmost layer of human and sustains most of the external UVB irradiation, which possibly causes the skin photoaging. As a natural antioxidant, marine natural products have been paid more and more attention to their positive effects on photoaging. 6,6′-bieckol is a phlorotanin isolated from Ecklonia cava, while its antiphotoaging bioactivity and mechanism have not been clear yet. This study proves that 6,6′-bieckol enhances cells viability and decreases the level of ROS in UVB-induced human immortalized keratinocytes (HaCaT) cells. It also resulted in significant downregulation of matrix metalloproteinases (MMPs), p-c-Fos, phosphorylated JNK, p38, IκB and p65. In addition, molecular docking also showed that 6,6′-bieckol could bind to MMP-1, MMP-3 and MMP-9. Finally, it was proved that 6,6′-bieckol acts on MMPs through the MAPK/AP-1 and NF-κB pathways to reduce UVB-induced oxidative stress damage in HaCaT cells. Therefore, 6,6′-bieckol is a functional food and skin care ingredient with great potential in preventing photoaging.