We studied the ontogeny of osmoregulation of Hemigrapsus sanguineus at an invaded area in the North Sea (islands of Helgoland and Sylt). This crab is native to the coasts of Japan and China but has successfully invaded the Atlantic coast of N America and N Europe. We reared larvae (zoea I to megalopa through five zoeal stages) and crab 1 at 24°C in seawater from hatching to intermoult of each developmental stage. At each stage, we exposed larvae and juveniles to a range of salinities (0-39‰) for 24h then we quantified haemolymph osmolality using nano-osmometry. We also quantified osmolality in field-collected adults after acclimation to the test salinities for 6 days. Larvae of H. sanguineus were able to hyper-osmoregulate at low salinities (15 and 20‰) over the complete larval development, although the capacity was reduced at the zoeal stage V; at higher salinities (25-39‰), all larval stages were osmoconformers. The capacity to hypo-regulate at high salinity occurred in crab 1 following metamorphosis. Adults were able to hyper-osmoregulate at low salinities and hypo-regulate at 39‰. H. sanguineus showed a strong capacity to osmoregulate as compared to its native competitor, Carcinus maenas, which only hyper-regulates at the first and last larval stages and does not hypo-regulate at the juvenile-adult stages. The capacity to osmoregulate over most of the life-cycle may confer a competitive advantage to H. sanguineus in areas were low salinity conditions are also characterised by high temperatures.