Richard Bell said the scorpion fell from the overhead bin and onto his head during lunch on a trip from Houston, Texas to Calgary in Canada last week.
After putting it on his plate, he was stung. United has offered compensation.
It happened on Sunday, on the same day a United passenger was violently dragged from a plane after refusing to give his seat to a staff member.
Video of the incident has been watched by millions of people online.
Dr David Dao, a 69-year-old Vietnamese-American, lost two front teeth and suffered a broken nose and a “significant” concussion in the incident.
Mr Bell, who was travelling with his wife, Linda, told CBC: “While I was eating, something fell in my hair from the overhead above me.
“I picked it up, and it was a scorpion. And I was holding it out by the tail, so it couldn’t really sting me then.”
A fellow passenger, he said, warned him that the creature was a scorpion and could be dangerous.
“So I dropped it on my plate and then I went to pick it up again, and that’s when it stung me. It got my nail, mostly,” he said
Mr Bell flicked the scorpion on to the floor and a flight attendant covered it with a cup before throwing it away in the bathroom.
A nurse who happened to be on board gave him a painkiller as a precaution, he said.
When the plane landed in Calgary he was taken to a hospital, and later released after being cleared of any medical issue.
Mr Bell said he had no plans to launch a lawsuit. United Airlines has offered the couple flying credit as compensation, CBC reports.
In the incident with Dr Dao on Sunday evening, law enforcement officials were called after he refused to leave the overbooked plane, saying he needed to get home to see his patients.
Dr Dao’s lawyers have filed an emergency court request for the airline to preserve evidence ahead of a hearing on Monday.
He was released on Wednesday night from a Chicago hospital, his lawyer said, adding that he planned to have reconstructive surgery.