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Homogeneous high aspect ratio cellulose nanofibrils (CNFs) were prepared from Laminaria hyperborea (LH) seaweed cellulose without any initial mechanical, biological or chemical pre-treatments. Fourier-transform infrared spectrophotometry revealed that LH cellulose was of the cellulose Iα allomorph, typical of algal cellulose. Compared with wood derived CNF, significant enhancements in crystallinity, viscoelastic properties, water retention values (WRV) and morphological characteristics were identified with a single pass at 1 wt. % cellulose content through a high-pressure homogeniser. Further mechanical fibrillation did not lead to appreciable improvements in material properties that would justify the added energy consumption, which at a single pass is at least a factor of 10 lower than with wood cellulose processing. Good quality CNFs with little compromise in material properties were also obtainable at 2–3 wt. % cellulose contents as identified from viscoelastic analysis, WRV and morphological analysis. LHCNFs also showed good thermal stability, which in summary presents a multifunctional high value cellulose nanomaterial that can find application in various fields. Graphic abstract