The black box recorder from Coast Guard Rescue Helicopter 116 has been recovered, according to the Air Accident Investigation Unit.
Naval Service divers have been examining the wreckage of Rescue 116 that crashed off the Mayo coast last week.
Dives to the site commenced this morning and continued throughout the afternoon when the black box was found.
The recorder was taken on board the Commissioners of Irish Lights' vessel, Granuaile.
It will be taken to Dublin before being brought to the UK. The AAIU said the recorder appears to be in very good condition with no external damage visible.
However it will have to be examined in the UK before it is clear if it has sustained any damage or not.
The black box contains both cockpit voice recordings and data on the history of the flight.
Conditions at sea were favourable today and search co-ordinators said they were focused on a key area of the wreckage.
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After detailed preparations over recent days, teams of specialist Naval Service divers travelled to the site just off Blackrock Island at first light and spent the day carrying out their task.
Operating at depths of 40 metres, they worked in pairs and dived for up to ten minutes at a time.
The search will resume in the morning.
The operation is being coordinated by LÉ Samuel Beckett, which is located close to the wreckage at Blackrock Island. It has support from local boats and the Granuaile.
The wreckage is located in waters 60m off the western face of Blackrock.
A three-mile exclusion zone is in place around the dive site and only vessels involved in the search are permitted to travel close to the area in question.
It is hoped that the three missing crew members - Captain Mark Duffy, Winch Operator Paul Ormsby and Winchman Ciarán Smith - may still be in the wreckage.
Detailed mapping of the undersea conditions has been carried out in recent days and images gathered by specialist cameras have been reviewed by the multiple agencies involved in the operation.
Specialist equipment was loaded onto boats at Blacksod pier yesterday evening in anticipation of today's favourable sea conditions.
All the agencies involved are stressing that safety concerns will take precedence throughout the operation.
At a briefing at Blacksod Lighthouse last night, Superintendent Tony Healy said the hope was that the three crew were in or near the wreckage.
He said the conditions for underwater exploration of the helicopter were the best that search teams had experienced so far.
Chief Investigator with the Air Accident Investigation Unit Jurgen Whyte said a lot of information had been gathered and that everything possible was being done to assist the dive process.
He said he was confident that dive conditions would be good for a number of days.
Mr Whyte said the priority was to recover the three crew members.