The Dublin-based aircraft was providing cover for another helicopter involved in an early morning rescue operation.
Contact was lost with the crew at around 1am.
A major sea search is ongoing off the Co Mayo coast - around six miles west of Blacksod.
Coast Guard helicopters from Sligo and Shannon along with the Air Corps maritime patrol aircraft Casa are involved.
They have been joined by lifeboats from Ballyglass and Achill and five local fishing vessels.
A Coast Guard spokesman said the missing helicopter had been providing cover in the evacuation of a crewman, requiring urgent medical attention, from a UK-registered fishing vessel approximately 150 miles west of Eagle Island in Co Mayo.
“Owing to the distance involved, safety and communication support, known as Top Cover, was provided by the second Coast Guard helicopter, the Dublin based R116,” he said.
“The search operation is being coordinated by the Coast Guard Rescue Coordination Centre in Malin Head.
“Both helicopters refuelled at Blacksod prior to transiting to the scene.”
More to follow