A stopper knot (or simply stopper) is a knot that creates a fixed thicker point on an otherwise-uniform thickness rope for the purpose of preventing the rope, at that point, from slipping through a narrow passage, such as a hole in a block.

It secures the end of a sewing thread; it provides a handhold or a foothold in bell ropes and footropes.

It adds weight to the end of a heaving line, and it is often employed decoratively, but it should not be used to prevent unlaying and fraying except in small cord, twine, and the like, as a whipping is in every way preferable for large and valuable material.A monkey's tail "is a permanent or semipermanent stopper that is put in the bight as well as the end.

The monkey's tail is preferred for the purpose just described because it does less damage to rope than any knot.

Common knots used for this purpose are: In nautical settings, a stopper may refer to a length of rope that is belayed at one end with the other end attached to a tensioned main line using a friction hitch in order to tension the stopper and thereby slacken the portion of the tensioned main line behind the friction hitch.