Protein by-products from Undaria pinnatifida (U. pinnatifida) sporophyll processing (i.e., after polysaccharide extraction) were hydrolysed using flavourzyme (enzyme activity = 7592 U/g). Optimal hydrolysis conditions were determined using response surface analysis (i.e., 7% flavourzyme for 18 h); a hydrolysate yield of 32.52 ± 0.46 g/100 g dry-solids was achieved with a degree of hydrolysis (DH) at 5.63 ± 0.27 g α-amino nitrogen/100 g total nitrogen. Five free amino acids (FAAs), alanine, glumatic acid, aspartic acid, proline, and glycine, were abundant in the best hydrolysate. Eighteen volatile flavour compounds were identified using gas chromatography/mass spectrometry, with hexanal, cedrol, nonanal, 2-heptenal, acetoin, and heptanal being the primary odorants. As indicated by the sensory panel, the hydrolysate from U. pinnatifida sporophyll by-products (HUPSB) exhibited an umami taste and a seaweed odour. It was concluded that the protein by-products of U. pinnatifida sporophyll processing could yield excellent seafood flavouring.