During development and adulthood, brain plasticity is evident at several levels, from synaptic structure and function to the outgrowth of dendrites and axons. Whether and how sex impinges on neuronal plasticity is poorly understood. Here we show that the sex-shared GABA (gamma-aminobutyric acid)-releasing DVB neuron in Caenorhabditis elegans displays experience-dependent and sexually dimorphic morphological plasticity, characterized by the stochastic and dynamic addition of multiple neurites in adult males. These added neurites enable synaptic rewiring of the DVB neuron and instruct a functional switch of the neuron that directly modifies a step of male mating behaviour. Both DVB neuron function and male mating behaviour can be altered by experience and by manipulation of postsynaptic activity. The outgrowth of DVB neurites is promoted by presynaptic neurexin and antagonized by postsynaptic neuroligin, revealing a non-conventional activity and mode of interaction of these conserved, human-disease-relevant factors.