Work will begin this summer to improve the island for its winged inhabitants, including creating a lagoon and roost sites for ducks and geese.
Whitton Island will form part of the Humber Wildfowl Refuge and will not be open to the public.
Pete Short, RSPB Humber Sites Manager, said: “The acquisition of Whitton Sands has given us a great opportunity to improve the protection for the internationally Humber Estuary’s important birds. It’s a great step towards ensuring the Estuary’s wildlife will thrive for generations to come. Combined with all our other reserves on the Humber we are proud to be able to provide a winter home and migration stopover for more than a 100,000 water birds.”