Barnacles are the only sessile crustaceans inhabiting intertidal zone, an extremely stressful environment for sessile organisms. Herein, we report the chromosome-level genome of a stalked barnacle, Capitulum mitella, which is a dominant intertidal cirripede of the west Pacific Ocean coast. After comprehensive comparative genomic analyses, it is the first time to find an ancient whole-genome duplication (WGD) event that preceded the divergence of Lepadomorpha and Sessilia approximately 237 million years ago. The retained duplicated genes of WGD are primarily enriched in many environmental information processing pathways, shedding light on its adaptive evolution of intertidal sessile life. In addition, transcriptomic and metabolomic sequencing and analyses of C. mitella indicated that the upregulation of some expanded anti-stress factors and accumulation of acyl-carnitines help barnacles adapt to stressful intertidal conditions. Therefore, this study provides a valuable resource for understanding the unique intertidal adaptation mechanism of sessile crustaceans and reveals novel WGD events in invertebrates.