The grey cat, named "Gin", broke out of its cage about 20 minutes after take-off and scurried to the cockpit where it scratched the co-pilot, SN Brussels Airlines said on Tuesday.
The flight with 62 crew and passengers had left Brussels for Vienna on Monday when the cat, a prized animal that travels to cat shows around the world, started wandering around the passenger cabin.
"The passenger was asleep and at that point the cat managed to escape the cage," an airline spokesman said.
The cat managed to slip through the cockpit door as a flight attendant served lunch to the pilots.
The scared animal was "very aggressive and scratched the co-pilot", forcing the crew to return to the airport, the spokesman said.
The passengers were put on another flight to Vienna, without the cat and its owner, who had to take a separate flight there.