The present invention relates to the cosmetic use of an extract of the plant Lythrum salicaria to maintain and / or increase the expression of collagen in the skin and / or mucous membranes to maintain and / or increase the firmness of the skin and / or mucous membranes and / or to maintain and / or increase lipolysis in the adipocytes of the skin to induce a slimming effect on the skin and / or to reduce, prevent and / or treat the unsightly manifestations of cellulite, such as orange peel appearance of the skin. Another subject of the invention relates to a cosmetic treatment method comprising the application, preferably topically, of the extract of Lythrum salicaria according to the invention or of a cosmetic composition comprising it for maintaining and / or increasing the expression of collagen in the skin and / or mucous membranes to increase the firmness of the skin and / or mucous membranes and / or maintain and / or increase lipolysis in the adipocytes of the skin to induce a slimming effect on the skin and / or to reduce, prevent and / or treat the unsightly manifestations of cellulite, for example the orange peel appearance of the skin. A final subject of the present invention relates to an extract of Lythrum salicaria for its use for the treatment and / or prevention of pathologies of the skin and / or mucous membranes involving loss of collagen expression and / or a loss of pathological firmness. such as those encountered in diseases such as rosacea or telangiectasia and / or involving dysfunction in lipolysis in adipose tissue. La présente invention concerne l'utilisation cosmétique d'un extrait de la plante Lythrum salicaria pour maintenir et/ou augmenter l'expression de collagène dans la peau et/ou les muqueuses pour maintenir et/ou augmenter la fermeté de la peau et/ou des muqueuses et/ou pour maintenir et/ou augmenter la lipolyse dans les adipocytes de la peau pour induire un effet amincissant au niveau de la peau et/ou pour diminuer, prév
