SummaryBackground/PurposeIn the present study, the effect of 3–5 kDa chitooligosaccharide (COS) on homeostasis between the expression of collagen‐degrading matrix metalloproteinases (MMPs) and collagen synthesis was investigated using ultraviolet (UV)‐A irradiated dermal fibroblasts.MethodsUV protection imparted by 3–5 kDa COS was measured by examining the UV absorption spectrum. Collagenase MMP secretion was examined using an enzyme‐linked immunosorbent assay. The levels of collagenases and collagen synthetic markers were determined by employing the reverse transcriptase‐polymerase chain reaction and Western blot analysis.ResultsThe 3–5 kDa COS not only absorbed UV‐A and UV‐B light but also inhibited collagenase (MMP‐1, MMP‐8, and MMP‐13) and gelatinase (MMP‐2 and MMP‐9) MMP expression. The suppression of MMP expression was found to be due to an increase in expression of the tissue inhibitors of MMP (TIMP)‐1 and TIMP‐2. Treatment with 3–5 kDa COS enhanced collagen synthetic markers such as procollagen, type I, III, and IV collagens in UV‐A‐irradiated dermal fibroblasts. Furthermore, the effects of 3–5 kDa COS on collagen degradation and collagen synthesis in UV‐A irradiated dermal fibroblasts were regulated via the inhibition of activating protein‐1 (AP‐1) signaling.ConclusionOur results suggest that 3–5 kDa COS can be used to develop as topical applications for antiphotoaging cosmeceuticals as it enhances collagen synthesis.