Dry rot is wood decay caused by one of several species of fungi that digest parts of the wood which give the wood strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resulted in a darkly colored deteriorated and cracked condition.

The life-cycle of dry rot can be broken down into four main stages. Dry rot begins as a microscopic spore which, in high enough concentrations, can resemble a fine orange dust. If the spores are subjected to sufficient moisture, they will germinate and begin to grow fine white strands known as hyphae. As the hyphae grow they will eventually form a large mass known as mycelium. The final stage is a fruiting body which pumps new spores out into the surrounding air.

In other fields, the term has been applied to the decay of crop plants by fungi. In health and safety, the term is used to describe the deterioration of rubber, for example the cracking of rubber hoses.