Small-scale robots, ranging in size from micrometers to centimeters, have gained significant attention in the biomedical field. However, conventional small-scale robots made of rigid materials encounter challenges in adapting themselves to the soft tissues and complicated environments of human body. Compared to the rigid counterpart, small-scale hydrogel-based robots hold great promises due to their tissue-like low modulus, outstanding biocompatibility and accessible stimuli-responsive capabilities. These attributes offer small-scale hydrogel-based robots with multimodal locomotion and reinforced functions, further enhancing the adaptability in manipulation and tasks execution for various biomedical applications. In this review, we present recent advances in small-scale hydrogel-based robots. We first summarize the design principles of small-scale hydrogel-based robots including materials, fabrication techniques and manipulation strategies, then highlighting their upgraded functions and adaptive biomedical applications. Finally, we discuss existing challenges and future perspectives for small-scale hydrogel-based robots.