The genomes of Brassica and Sinapis share evidence of the ancestral Whole-genome Triplication (WGT) event that distinguishes tribe Brassiceae from other lineages within the family Brassicaceae. This is validated through high-quality genome assemblies of Brassica nigra, Sinapis alba and S. arvensis that also reveal that Sinapis may represent the more ancient lineage. This study provides new insights into genome evolution and phylogenetic relationships within the Brassiceae and provides genomic information for genetic improvement of these plants.