Stiffness variable materials have been applied in a variety of engineering fields that require adaptation, automatic modulation, and morphing because of their unique property to switch between a rigid, load‐bearing state and a soft, compliant state. Stiffness variable polymers comprising phase‐changing side‐chains (s‐SVPs) have densely grafted, highly crystallizable long alkyl side‐chains in a crosslinked network. Such a bottlebrush network‐like structure gives rise to rigidity modulation as a result of the reversible crystallization and melting of the side chains. The corresponding modulus changes can be more than 1000‐fold within a narrow temperature span, from ≈102 MPa to ≈102 kPa or lower. Other important properties of the s‐SVP, such as stretchability, optical transmittance, and adhesion, can also be altered. This work reviews the underlying molecular mechanisms in the s‐SVP's, discusses the material's structure–property relationship, and summarizes important applications explored so far, including reversible shape transformation, bistable electromechanical transduction, optical modulation, and reversible adhesion. Polymers comprising side chains undergoing reversible crystallization and melting in a narrow temperature span have been synthesized for stiffness control. A modulus change of more than 1000‐fold can be achieved. Larger stiffness changes can be obtained. The phase‐changing can additionally be exploited for reversible adhesions, bistable actuation, and opacity modulation. Important applications will be discussed.