The flight, which took off from Tenerife bound for Glasgow Prestwick airport on Sunday, returned to the airport after the co-pilot, or first officer, fell ill.
An alert was made by the plane just 13 minutes into the flight, when it was travelling at an altitude of around 25,000 feet and a speed of over 450 miles per hour.
The flight then performed a mid-air U-turn and returned to Tenerife South Airport, where it was met by local emergency services.
The condition of the co-pilot is not yet known.
No fuel dumping or burning was required, as can sometimes happen when a plane is forced to return to an airport or make an unscheduled landing.
International flights have a minimum of two pilots on board, and they are trained to be able to operate the aircraft alone if one of them becomes incapacitated.
A spokesperson for Ryanair told the Daily Mirror: “This flight from Tenerife to Glasgow (April 7th) returned to Tenerife Airport shortly after take-off after the first officer became ill inflight.
“In line with procedures, the captain contacted ATC, the aircraft landed normally, and the first officer was met by medical personnel.
“Customers disembarked and departed to Glasgow shortly afterwards.”