The brain is capable of massively parallel information processing while consuming only similar to 1-100 fJ per synaptic event(1,2). Inspired by the efficiency of the brain, CMOS-based neural architectures(3) and memristors(4,5) are being developed for pattern recognition and machine learning. However, the volatility, design complexity and high supply voltages for CMOS architectures, and the stochastic and energy-costly switching of memristors complicate the path to achieve the interconnectivity, information density, and energy efficiency of the brain using either approach. Here we describe an electrochemical neuromorphic organic device (ENODe) operating with a fundamentally different mechanism from existing memristors. ENODe switches at low...