ABSTRACT 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. 6,6′‐bieckol, as a polyphenol from Ecklonia cava with strong antioxidant capacity, is effective to reduce UVB‐induced ROS formation by downregulating MAPK/AP‐1/NF‐κB pathways in HaCaT cells. Due to the phenolic hydroxyl properties in its structure, 6,6′‐bieckol is possible to act inhibition to MMPs activities and decrease collagen degradation further.