The passenger was attacked by the eight-legged arachnid while travelling from San Francisco to Atlanta.
She told the TMZ website she started to feel a stinging sensation on her leg after the plane took off on Thursday morning.
When she went to the toilet to investigate the pain, a scorpion fell out of her trousers and scurried away.
The passenger was taken to hospital after landing but is expected to make a full recovery.
The arachnid, meanwhile, was captured by flight attendants and put in a box. No further details were given about its condition.
United Airlines said in a statement: “After learning that one of our customers on flight 1554 from San Francisco to Atlanta was stung during flight, our crew responded immediately and consulted with a MedLink physician on the ground who provided medical guidance.
“Upon landing in Atlanta, the flight was met by medical personnel and the customer was transported to a local hospital.
“We have been in contact with our customer to ensure her wellbeing.”
Earlier this year a large scorpion was seen crawling out of the overhead lockers on a Lion Air flight in Indonesia.
And two years ago another United Airlines passenger told how he was stung by a scorpion on a flight from Houston in Texas to Calgary in Canada. Passenger Richard Bell said the arachnid fell from the overhead luggage rack onto his head.