The motions of the heart are regulated by electrophysiological signals, which can be monitored and altered by bioelectronic devices for the diagnosis and treatment of cardiovascular diseases. In particular, soft bioelectronic devices, composed of deformable and conductive materials, can be implanted or designed as wearable devices to enable conformal contact with heart tissue or the skin for real-time and precise diagnosis and treatment. In this Review, we discuss the design and materials of soft bioelectronic devices, highlighting their advantages, as compared with rigid bioelectronic devices, in cardiovascular disease management. We examine the engineering and applications of soft implantable bioelectronics, including cardiac mapping devices, cardiac stimulation devices and mechanically assisting devices, as well as wearable soft bioelectronics, such as blood oxygen saturation sensors, heart monitoring devices and transdermal drug delivery systems. Finally, we outline technical challenges and future opportunities for the clinical application of soft bioelectronic devices, such as a wireless power supply, optogenetic control of cardiac motion, bioadhesives for device–tissue interfaces and artificial intelligence-assisted data analysis.