The Spaniard, twice world champion, hit the wall on the exit of Turn Three at speeds of up to 150 miles per hour.
An ambulance was dispatched to treat the 33-year-old Alonso at the scene of the accident.
He was taken to the circuit's medical centre for initial treatment before being airlifted to hospital for precautionary checks.
"Fernando Alonso crashed in the end of Turn Three. The car has gone back and crashed into the wall," a statement from the circuit said.
The Circuit de Catalunya tweeted: "It seems that Alonso is ok. We hope that everything's alright and all ends with just a shock!!"
It is unsure whether the crash was a result of a driver error or a malfunction with the car.
Alonso's former Ferrari team, whom he left at the end of last season, tweeted; "All the best Fernando."
Image: Fernando Alonso of Spain and McLaren Honda receives medical assistance after crashing
Photograph: Mark Thompson/Getty Images