The flexural rigidity and bending modulus of kelp, Laminaria hyperborea, collected at the MarGate area (https://www.awi.de/en/science/special-groups/scientific-diving/margate.html) north of Heligoland, Germany (latitude: 54° 11.700'N, longitude: 7° 52.600'E) was determined from measurements performed at the Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research. Scientific divers from the Biological Institute Helgoland, AWI, collected nine kelps (Laminaria hyperborea) from the MarGate area on 21.06.2022. The collected kelps were transported into the laboratory in boxes filled with seawater from the site and stored in laboratory sinks filled with running aerated seawater from the North Sea during the experiments. The measurements were carried out on 23.06.2022, 25.06.2022, and 27.06.2022. They consisted of cutting strips 20 cm in length (L) and 2.5 cm in width (b) from the blades close to the stipe of each kelp. The cut-out strips were towel-dried, and their thickness (t, mm) and weight in grams were measured. The weight in grams was converted to weight per unit area (w, N/m²) to compute the flexural rigidity per unit width (J, Nm). A standard ruler with precision for the nearest millimeter was used to measure the length (L), width (b), and cantilever length (l) of strips. The thickness (t) of the strip was measured with a caliper gauge that measured to the nearest 0.01 millimeter. The weight of the strip was measured by a weighing scale (Sartorius, LE323S), which had a precision of 0.001 grams. The cut-out strips from each kelp form the nine samples tested for the bending properties. Each sample is used to repeat the cantilever test four times, i.e., both sides' ends, as Henry (2014) recommended to improve the accuracy. An apparatus consisting of two planes, one angled at 45° (θ = 45°) and the other parallel to the horizontal, was used for the test. The device was clamped onto a table on the horizontal plane. The experimental protocol consists of laying each strip onto the apparatus with the strip's edge coinciding with the apparatus's angled edge. After that, the strip is slowly moved forward with a ruler, with the ruler's zero coinciding with the strip's edge. This is done until the tip of the strip touches the inclined plane. The horizontal projection of the length of the hanging strip is equal to the distance between the ruler's tip and the apparatus's angle, termed the cantilever length (l). The flexural rigidity per unit width (J, Nm) and the bending modulus (Eb, N/m²) are then calculated with the second moment of area (I, m⁴) as in Henry (2014).