The ascidian neural plate exhibits a regular, grid-like arrangement of cells. Patterning of the neural plate across the medial–lateral axis is initiated by bilateral sources of Nodal signalling, such that Nodal signalling induces expression of lateral neural plate genes and represses expression of medial neural plate genes. One of the earliest lateral neural plate genes induced by Nodal signals encodes the transcription factor Snail. Here, we show that Snail is a critical downstream factor mediating this Nodal-dependent patterning. Using gain and loss of function approaches, we show that Snail is required to repress medial neural plate gene expression at neural plate stages and to maintain the lateral neural tube genetic programme at later stages. A comparison of these results to those obtained following Nodal gain and loss of function indicates that Snail mediates a subset of Nodal functions. Consistently, overexpression of Snail can partially rescue a Nodal inhibition phenotype. We conclude that Snail is an early component of the gene regulatory network, initiated by Nodal signals, that patterns the ascidian neural plate.