Circular RNAs (circRNAs) are covalently closed RNA molecules that regulate various biological processes in plants. However, the functions of most identified circRNAs remain unclear. Here, we report a nucleoplasmic-localized circRNA, Vv-circRCD1, derived from exons 2 and 3 of the grape VvRCD1 gene. Overexpression of Vv-circRCD1 significantly shortened primary root length and increased root hair number and length, notably, and improved the salt tolerance in Arabidopsis. Transient overexpression also significantly enhanced salt tolerance of grapevines. In silico analyses confirmed direct sequence complementarity between Vv-circRCD1 and the Vvi-miR399 family, and Vv-circRCD1 and Vvi-miR399 target genes (involved in salt stress responses) showed consistent expression patterns under salt stress, indicating a Vv-circRCD1–Vvi-miR399–target gene regulatory module may mediate salt tolerance. These results not only identified Vv-circRCD1 as a novel regulator of grapevine salt tolerance, but also highlighted its potential in improving crop stress resistance, providing a practical reference for crop breeding.