Ultrasound-assisted extraction (UAE) of polyphenols from the brown seaweeds Ascophyllum nodosum leaves a solid phase where alginates can be extracted. This study characterizes alginates extracted after the UAE process, with and without an intermediate drying stage at different temperatures (50 and 90 °C) producing sequentially two bioactive compounds from a unique raw material. FT-IR and ¹H NMR analyses showed the high purity of alginates with features in the range of commercial alginates. Drying at high temperature decreased average block length and viscosity average molecular weight (M_v) of alginate from 428 to 133 kg/mol. Steady-shear curves (shear-thinning behaviour) and viscoelasticity (liquid like character) features depended clearly on M_v. Solutions of alginates with high M_v were more viscous and the elastic character was more relevant. Cox-Merz rule was only accomplished within the semi-dilute regimes of alginate concentration. Tested process conditions allow the production of alginates with different properties.