Speaking in No 10, Mr Cameron said the Argentines should take "careful note" of the virtually unanimous outcome.
"The Falkland Islands may be thousands of miles away but they are British through and through and that is how they want to stay. People should know we will always be there to defend them," he said.
"The Falkland islanders couldn't have spoken more clearly. They want to remain British and that view should be respected by everybody, including by Argentina."
The vote was organised against a back-drop of increasingly strident assertions of Argentina's claims to the islands by President Cristina Fernandez de Kirchner. The government in Buenos Aires insists that the referendum of what it calls an "implanted people" is "pointless" and has no validity in law.
Alicia Castro, the Argentine ambassador to London, said the referendum was "totally irrelevant" and "a manoeuvre with no legal value".