The alarm was raised at about 11am, according to the Maritime and Coastguard Agency (MCA).
The diver, who was part of a group, was reported missing when they failed to resurface by the dive vessel they were using close to Kinlochbervie.
Police Scotland, coastguard rescue teams from Kinlochbervie and Melness, a helicopter and the Lochinver RNLI lifeboat have all been deployed as part of a multi-agency response.
An MCA spokesperson said: "A mayday call has been put out to any nearby vessels to request their assistance in a search.
"The coastguard emergency tow vessel, Ievoli Black, has also joined the search effort further offshore."
It comes after the death of David Pleace, aged 57, a diver from Derby, Orkney, who had been reported missing Sunday, 27 June.
His body was found on Monday.