Abstract: Provided are an assistance device, an assistance garment, and an assistance method that enable easy replacement of a part of structure, suppression of fatigue of a wearer, and reduction of a load in an action of the wearer, without requiring time and effort for putting on and taking off. An assistance device 1 is art assistance device 1 that is provided on a garment and reduces a load in an action of a wearer 5. The assistance device 1 is provided with an assisting part 10 that is replaceable and applies force for assisting the an action to the body of the wearer 5 by changing flexibility, and an assist-control part 12 that controls the flexibility.