Liverpool Bay has been designated a Special Protection Area (SPA) for birds.
The bay, which stretches from Anglesey in Wales to the Lancashire coast, is one of 15 marine protected areas to be given the status on Friday.
It is known for being home to thousands of common scoters and hundreds of red-throated diver birds each winter.
The bay has been given SPA status after being identified as internationally important for its biodiversity.
The special protection will means the wildlife has to be protected for the benefit of present and future generations, the Countryside Council for Wales said.
Nearly 1,000 red-throated divers spend the winter in Liverpool Bay.
And more than 50,000 common scoter sea ducks - 58% of Britain's population - regularly make the bay their home every winter.
They are often seen in large groups offshore, or as long lines flying along the coast.
The decision to grant the bay SPA status, under the European directive on the conservation of wild birds, follows a three-month public consultation, which started last November.