The man screamed when he saw the arachnid clinging to his bag at Victoria Underground station in central London.
Staff were quickly on hand after hearing the commotion, British Transport Police (BTP) said. They managed to capture the scorpion and placed it in a plastic lunchbox, which they pierced with air holes, while they called the police.
Specialist wildlife crime officers from BTP went to the station to remove the creature, which had been put on a shelf in the station manager's office. PC Mike Charnick, who dealt with the incident, said: "I contacted London Zoo and made arrangements to take the animal to be assessed."
Keepers safely housed it and experts at the Natural History Museum identified the species as a Centruroides, which may have come from the Caribbean.
Dave Clarke, bugs team leader at ZSL London Zoo, said: "It is bizarre that this species was found in somewhere like Victoria station. We have been keeping the arachnid safely and securely whilst establishing an exact identification of the creature.
"The scorpion really is a beautiful animal and it's a shame that it has been exposed to such a stressful situation."
The passenger who discovered the scorpion told staff he had been sitting in a park all day and had not been abroad recently.
He did not leave a name or any contact details so officers have been unable to work out where he picked it up.
No passengers were at risk during the incident, which happened in the evening of Sunday April 6, BTP said.
Pc Charnick added: "Our job is to make the railways safe for the travelling public, and that includes possible dangers posed by animals."