In biointegrated electronics, the facile control of mechanical properties such as softness and stretchability in electronic devices is necessary to minimize the perturbation of motions inherent in biological systems1-5. For in vitro studies, multielectrode-embedded dishes6-8 and other rigid devices9-12 have been widely used. Soft or flexible electronics on plastic or elastomeric substrates13-15 offer promising new advantages such as decreasing physical stress16-18 and/or applying mechanical stimuli19,20. Recently, owing to the introduction of macroporous plastic substrates with nanofibre scaffolds21,22, three-dimensional electrophysiological mapping of cardiomyocytes has been demonstrated. However, quantitatively monitoring cells that exhibit significant dynamical motions via electric probes over a long period without affecting their natural motion remains a challenge. Here, we present ultrasoft electronics with nanomeshes that monitor the field potential of human induced pluripotent stem cell-derived cardiomyocytes on a hydrogel, while enabling them to move dynamically without interference. Owing to the extraordinary softness of the nanomeshes, nanomesh-attached cardiomyocytes exhibit contraction and relaxation motions comparable to that of cardiomyocytes without attached nanomeshes. Our multilayered nanomesh devices maintain reliable operations in a liquid environment, enabling the recording of field potentials of the cardiomyocytes over a period of 96 h without significant degradation of the nanomesh devices or damage of the cardiomyocytes.