Exopolysaccharides (EPS) are polysaccharides produced and excreted by microorganisms, such as bacteria and microalgae. Unlike other microorganisms, studies on EPS from microalgae are scarce. The present work has taken part in the ANR POLYSALGUE project (figure 1), whose purpose is to increase the current knowledge regarding microalgae EPS and their valorisation. In this context, we aimed at investigating biological activities of 6 new EPS from microalgae in the field of cosmetics [1]. The first part of this work consisted in the development of a supported depolymerization method, in order to reduce the molecular weight of these polymers. Depolymerized EPS were biochemically characterized, analysing their molecular weight by SEC-MALS/RID/VD methods, and levels of sulfate groups, uronic acids and proteins were determined. The second part was the evaluation of the anti-aging potential of these EPS. For this purpose, in vitro productions of collagen and hyaluronic acid were quantified on cell models of human dermal fibroblasts from 20-years and 46-years old female donors. A principal component analysis highlighted a correlation between the anti-aging activity of EPS and their uronic acid levels. Enzymatic kinetic analyses of metalloproteases and hyaluronidases were also conducted, indicating that the inhibition of metalloproteases seemed to be correlated with sulfate groups content of EPS. Finally, an analysis of transcripts showed that the increase in the collagen produced by fibroblasts could be due to the activation of the TGF-β1 signalling pathway and the inhibition of metalloproteases biosynthesis.