Organic crystals, possessing both flexibility and stimuli-responsive properties, are attractive candidates as smart materials, namely sensors, actuators, optoelectronic devices, information storage, medical applications, etc. They are emerging as a new research direction in materials science, namely, crystal adaptronics. For a long time, it has been generally considered that crystals are inclined to crack upon external force, which limits their application as elements. Recently, however, more and more crystals that display excellent deformation have been reported, which sparks interest in their potential applications as stimuli-responsive flexible smart materials. This comprehensive review summarizes these remarkably flexible crystals with stimuli-responsive (light, thermal, electrical and multiple) properties reported to date, delves into their response mechanisms and structure–property relationships, and discusses the design strategies for endowing flexible organic crystals with stimuli-responsive properties. Finally, the challenges and future outlook in this field are discussed.