Grillage foundations may be chosen as an alternative to standard shallow offshore foundations owing to their improved hydrodynamic performance during installation. They consist of a mesh of vertical grilles that penetrate the seabed. During vertical loading, the capacity of a conventional surface foundation may be obtained from grillage foundations if sufficient penetration is permissible. In this paper, a series of laboratory tests is reported to investigate the performance of grillage foundations in sand with different spacing, grille thickness, and numbers of grilles. The load-displacement results are examined in detail and compared with a previously proposed analytical solution. It is shown that the analytical solution captures the key aspects of the foundation's performance if minor improvements are made to the analysis. It is proposed that the improved analytical solution may form the basis of future design. In addition, intergrille plugging behaviour similar to that observed for open-ended piles has been examined photographically.