What type of energy is stored in a stretched rubber band?

The energy stored in a stretched rubber band is elastic potential energy.

Stretching a rubber band requires work: the applied force changes the arrangement of its polymer chains and stores energy in the material. When the band is released, that stored energy can become kinetic energy, sending it across a room or powering a simple toy. The same principle operates in bows, spring toys, trampolines, and mechanical springs.

For an ideal spring obeying Hooke's law, elastic potential energy is written as ½kx², where k is stiffness and x is displacement from the relaxed position. A rubber band is more complicated than an ideal spring because its force does not remain perfectly proportional to extension, especially over large stretches.

Rubber bands also lose some energy as heat through internal friction, a behavior called hysteresis. That is why a real band may not return exactly the energy used to stretch it. Its stored energy is not chemical energy, even though rubber's molecular structure affects how it responds.