Cellulose was successfully extracted from Ulva lactuca, an abundant and unexploited green macroalga that causes several environmental disorders by its excessive eutrophication. The extracted cellulose purity was checked by acid hydrolysis and high performance anion exchange chromatography coupled to pulsed amperometric detection, showing mainly glucose, and by Fourier transform infrared spectroscopy analysis, revealing the same functional groups as the commercial cellulose AVICEL and the absence of uronic esters from hemicellulosic contaminants. The crystallinity index was 59% compared to 82% for the AVICEL cellulose, showing the semi crystalline character of the extracted cellulose. The thermal behavior of both celluloses was the same as they showed a degradation peak at 360 °C as demonstrated by the thermogravimetric analysis. Beside the physicochemical properties, the alga cellulose presented several interesting rheological properties such as a high specific surface area (5.74 m2/g of dry matter) and consequently a higher water retention (43.07 g/100g of dry matter). These results show that alga can be an alternative source of cellulose to the conventional ones such as wood.