Sinojackia Hu. comprises five to eight Chinese endemic species with high ornamental and medicinal value. However, the generic limits, interspecific relationships and evolutionary history of the genus remain unresolved. In this study, we newly sequenced three plastomes of S. oblongicarpa and compared them with those of the other congeneric species to explore the taxonomic delimitation of the species and the evolutionary history of the genus. The plastome structure of Sinojackia species was extremely conserved in terms of number of genes, sequence length, and GC content. The codon usage patterns revealed that natural selection may be the main factor shaping codon usage bias. Our phylogenetic tree shows that Sinojackia is monophyletic and can be divided into two clades. Sinojackia oblongicarpa as a distinct species is supported for it is distantly related to S. sarcocarpa. The evolutionary analysis of morphological features indicates that the woody mesocarp is an ancestral feature. Sinojackia originated in central Southeast China during the early Miocene. In this period, it experienced elevated diversification and migrated from central Southeast China to the Hunan Province and the Sichuan Province with the development of the Asian monsoon and East Asian flora. Glacial–interglacial interactions with the monsoon climate may provide favorable expansion conditions for Sinojackia on a small scale.