The aim of this study is to investigate the relationship between the structural, molecular, and particulate properties of alginic acid and its functional characteristics in direct compression (tabletability, compressibility, elasticity, deformation mechanism, and disintegration ability). Therefore, accurate characterization of two different batches of alginic acid was executed (X-ray powder diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electronic microscopy, 1H nuclear magnetic resonance, size exclusion chromatographymulti angle light scattering, viscosimetry, carboxylic acid titration, powder flowability, true density, laser granulometry). Results showed that molecular weight seems to affect tablet properties and that the alginic acid with the lowest molecular weight provides the hardest tablets with the lowest elastic recovery. Furthermore, these results show the potential interest of exploiting alginic acid as filler excipient in tablet formulation. Finally, disintegration properties of tested materials were found to be close to that of commercial superdisintegrants (Glycolys® and Kollidon Cl®) but not correlated to their swelling force. It can be concluded, for the first time, that the determination of alginic acid molecular weight seems key for applications in direct compression and in particular for obtaining tablets with reproducible strength.