The Easyjet Airbus A319 from Nice to Luton decalred the emergency at around 11.15am.
The aircraft diverted to London's Gatwick Airport where it landed safely.
An Easyjet spokesman said: “easyJet can confirm that flight EZY2122 from Nice to London Luton on Monday 21st May diverted to London Gatwick due to a passenger onboard requiring urgent medical assistance.
“The safety and wellbeing of our passengers and crew is easyJet’s highest priority. The passenger was taken to hospital and the flight continued to London Luton.
“We apologise for any inconvenience experienced due to the diversion and resultant delay.”