Self-avoidance allows sister dendrites from the same neuron to form non-redundant coverage of the sensory territory and is important for neural circuitry functions. Here, we report an unexpected, cell-autonomous role of the Wnt-secretory factor MIG-14/Wntless in mediating dendrite self-avoidance in the C. elegans multidendritic PVD neurons. Similar findings in Drosophila suggest that this novel function of Wntless is conserved. The mig-14 mutant shows defects in dendrite self-avoidance, and ectopic MIG-14 expression triggers dendrite repulsion. Functions of dendrite self-avoidance and Wnt secretion could be mapped to distinct MIG-14 domains, indicating that these two functions of MIG-14 are genetically separable, consistent with lack of self-avoidance defects in the Wnt mutants. We further demonstrate that MIG-14 engages Wiskott-Aldrich syndrome protein (WASP)-dependent actin assembly to regulate dendrite self-avoidance. Our work expands the repertoire of self-avoidance molecules and uncovers a previously unknown, Wnt-independent function of MIG-14/Wntless.