A guyed mast or guyed tower is a tall thin vertical structure that depends on guy lines (diagonal tensioned cables attached to the ground) for stability. The mast itself has the compressive strength to support its own weight, but does not have the shear strength to stand unsupported. It requires guy lines to stay upright and to resist lateral forces such as wind loads. Guy lines are usually spaced at equal angles about the structure's base.

Guyed masts are used for telecommunications, sailing, and meteorology. The tallest guyed mast in the world is currently the 2,063 feet (629 m) KVLY-TV mast near Blanchard, North Dakota, USA.

Two subtypes exist. A partially guyed tower is a structure consisting of a guyed mast on top of a freestanding tower. The guys may be anchored to the top of the freestanding structure, or to the ground. A famous tower of this type is the Gerbrandy Tower. An additionally guyed tower is a freestanding tower which either has guys attached temporarily to add stability, for example during construction, or guys attached in only one direction to support unidirectional shear stresses. An example of the latter type is a utility pole at the end of a power line where the line ends or angles off in another direction. The pole requires guys in only one direction to support the unbalanced lateral load of the power line in the other direction.