AbstractReproducing human nervous systems with endogenous mechanisms has attracted increasing attention, driven by its great potential in streamlining the neuro‐electronic interfaces with bilateral signaling. Here, an artificial aquatic autonomic nervous system (ANS) with switchable excitatory/inhibitory characteristics and acetylcholine (ACh)‐mediated plasticity is reported based on the newly emerged organic photoelectrochemical transistor (OPECT). Under the modulation of spatial light and ACh, the system exhibits an immediate switch between excitation and inhibition, and many pulse patterns as well as advanced ANS functions are mimicked. To demonstrate its potential usage, the artificial ANS is then utilized to control artificial pupils and muscles to emulate real biological responses during an emergency. In contrast to previous solid‐state attempts, this ANS is aqueous compatible just like biological nervous systems, which are capable of real neurotransmitter mediation.