The discovery comes two days after the airliner's flight data recorder was also retrieved from the ocean.
Both recorders are said to be in good physical condition.
The Airbus A330 was flying from Rio de Janeiro in Brazil to Paris when it went down on 1 June 2009, killing all 228 people on board.
"We can now hope to find out what truly happened within the next three weeks," French Transport Minister Thierry Mariani told French radio.
France's Bureau of Investigation and Analysis (BEA) said the so-called black box would be shipped back to France, probably by the end of next week.
"The outside appears to be in relatively good shape," a spokeswoman said, adding it would be possible to see if the recorder was usable only once it was opened in France.Long search
Last week, the French search team found the outer casing and then the memory unit of of the flight data recorder. The crucial unit was located on Sunday and delivered to a search vessel.
The wreckage was discovered last month after a long search of 10,000 sq km (3,860 sq miles) of sea floor.
Air France flight 447 went down after running into an intense high-altitude thunderstorm, four hours following take-off from Rio de Janeiro.
Those on board came from more than 30 countries, though most were French, Brazilian or German.
Experts say the data in the two flight recorders is the only hope of finding out why the plane crashed into the sea.
The cockpit voice recorder - one of the so-called "black boxes" - contains vital information about the pilots' reactions.
The flight data recorder collects in-flight data from the plane's instruments, such as altitude, speed and rudder position.
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