Acorales is the sister lineage to all the other extant monocot plants. Genomic resource enhancement of this genus can help to reveal early monocot genomic architecture and evolution. Here, we assemble the genome ofAcorus gramineusand reveal that it has ~45% fewer genes than the majority of monocots, although they have similar genome size. Phylogenetic analyses based on both chloroplast and nuclear genes consistently support thatA. gramineusis the sister to the remaining monocots. In addition, we assemble a 2.2 Mb mitochondrial genome and observe many genes exhibit higher mutation rates than that of most angiosperms, which could be the reason leading to the controversies of nuclear genes- and mitochondrial genes-based phylogenetic trees existing in the literature. Further, Acorales did not experience tau (τ) whole-genome duplication, unlike majority of monocot clades, and no large-scale gene expansion is observed. Moreover, we identify gene contractions and expansions likely linking to plant architecture, stress resistance, light harvesting, and essential oil metabolism. These findings shed light on the evolution of early monocots and genomic footprints of wetland plant adaptations.