A trundle bed is a type of bed which is built on casters so that it can be slid underneath another bed or piece of furniture. Usually a main bed and a trundle bed are sold together as a set, to ensure that the trundle bed will fit underneath and be properly supported. A trundle bed can be an excellent space saving solution as well as a convenient way to have a guest bed available when you need it, but out of the way when you do not.

Often a trundle bed assembly is used as a day bed. The primary bed resembles a small couch and can be used as a piece of living room or lounging furniture. When the bed function is needed, bedding can be moved around to create a bed, and when an extra place to sleep is necessary, the trundle bed can be pulled out. In some cases, the trundle bed is designed to pop up to the same height as the day bed, creating a double bed for guests. Some trundle bed assemblies are also designed with drawers to hold bedding and pillows.

Trundle beds are sometimes used in children's rooms, to provide a bed for a sleepover guest. A trundle bed can also be useful when multiple children share the same room, allowing them to be active during the day and pull the bed out at night. Most trundle beds are light enough for a child to move without the assistance of an adult, meaning that the child can make the bed and put it away when he or she gets up in the morning.

In most cases, because a trundle bed is designed to fit into a small space, the mattress will be very thin. This can be uncomfortable for older guests or people with back problems, who often require more support while they sleep. If this is expected to be an issue, a specially designed trundle bed can be purchased, although it will force the height of the primary bed up.

Trundle beds can be made from a wide variety of materials to complement many interior decorating schemes. Common trundle/daybed assemblies include ones carved from wood, made from cast iron, or upholstered in fabric to resemble traditional couches. The price for a trundle bed set can vary widely, depending on what type of materials are used and the quality of the mattresses.