Nowadays, seaweeds play a grand role in digestive health because they are a valuable source of dietary fibers, especially insoluble fibers that have potential industrial applications. This study aims to evaluate the insoluble fiber content of three seaweed species classified as brown, green, and red algae using the acid detergent fiber (ADF) method with 0.5 M H2SO4. The outcomes demonstrated significant variations in fiber content among species, with brown algae displaying the highest levels. Fucus vesiculosus, as a brown algae, was found to have the highest fiber content at 42.73%. Across the algae, red algae Prophyra dioica showed an average fiber content of 36.24%, while the green algae Ulva Lactuca contained 33.85% fibers. Statistical analysis was conducted with Python version 3.9 software, incorporating the NumPy and SciPy libraries. A two-way analysis of variance was performed for each experiment to determine the differences between the experimentally acquired ADF values and those reported in the literature, highlighting inconsistencies in fiber quantification methods. These discrepancies demonstrate the superiority of using consistent measurement techniques and underscore the need for further investigation into the functional properties of seaweed fibers. The ADF method for insoluble fiber was validated by direct comparison with the standard AOAC enzymatic–gravimetric dietary fiber procedure on the same seaweed samples, obtaining less than a 5% difference between the AOAC enzymatic–gravimetric and the ADF method. The high-level fiber content in brown algae illustrates them as favorable candidates for the development of functional foods and other industrial applications, in addition to emphasizing the potential of seaweeds as sustainable sources of dietary fiber.