The chemical profile of biorefined Saccharina latissima, Ascophyllum nodosum and Palmaria palmata after carbohydrate and polyphenol extraction was analysed with the aim to evaluate the nutritional aspects of biorefined seaweeds as a novel animal feed supplement. Optimised enzymatic saccharification has been used to show that the protein concentration in the residue of P. palmata and A. nodosum can be increased by more than 2-fold. Nutritional value of the residue was further enhanced through an increase in total amino acids and fatty acids. As a consequence of removal of inorganic elements such as sodium, potassium and chloride, the total solid and ash content of all three seaweeds was reduced by around 40%. In contrast, divalent metals such as iron and zinc, as well as silicon, accumulated in all three residues. Potentially harmful components such as arsenic and iodine were reduced only in brown biorefined seaweeds, whilst in biorefined P. palmata, iodine increased by 39% compared to a 24% decline of arsenic. Nutritional values such as total fatty acid and total amino acid content increased in all three seaweeds after enzymatic saccharification. Polyphenol removal in all three seaweeds was >80% using aqueous acetonitrile and, in combination with enzymatic saccharification, did not impact on protein recovery in A. nodosum. This highlights the potential of biorefinery concepts to generate multiple products from seaweed such as extracts enriched in polyphenols and carbohydrates and residue with higher protein and lipid content.