The security barrier (1) is formed of a plurality of modular security bulwarks (2) each adapted for removable attachment to the peripheral edges of a ship such as, but not limited to, a ship's rail. Each security bulwark consists of a skirt section (3) which lies adjacent to and substantially parallel with a ship's wall or railing a head section (4) and a rear wall (10). The head section (4) has a generally smooth outer surface that slopes downwardly and outwardly from its uppermost point (5) so as to form an overhang (6) which projects outwardly from the ship's railing with a cross-sectional depth greater than the reach of a convention roofing ladder. The security barrier (1) is able to deter unauthorized access to the ship by preventing the use of conventional roofing ladders and the overhang in combination with the smooth surface of the security bulwark make it difficult for an individual to climb over even with grappling hooks.