The holidaymaker, who wished to remain anonymous, said she is confident she saw the child in Morocco on May 9, 2007, six days after the three-year-old vanished.
Just thinking about the sighting as the 10-year anniversary of her disappearance looms makes her feel sick, she said.
She told Daily Star Online: "I was in Marrakech at a petrol station and I saw this man with a little girl.
The woman claims she saw Madeleine McCann in Morocco
"She looked very distressed. She said 'can we go see mummy now?'
"It was English – I can't speak many other languages."
Several theories have surrounded Maddie's disappearance from the holiday apartment in Praia da Luz in 2007 while her doctor parents, Kate and Gerry, were dining nearby with friends.
One of those is she was abducted by child traffickers who immediately whisked her out of Portugal.
The woman, who lives in Spain, said the man she saw with the child was in his thirties and roughly 5ft 10ins tall.
She said he was not Moroccan and the child did not look like him.
Maddie went missing in 2007
Get Quotes on Home Insurance
She added: "I think that was what worried me.
"I was talking to my husband on the way back in the car and saying there was something wrong.
"That's what worried me a lot. But I didn't recall any connection with the case until I got home…
"I phoned the police, the missing number, everybody."
Madeleine McCann: The search continues
Fri, February 24, 2017
These are the various computer generated images of how Maddie might look now
1 of 14
Madeline McCann went missing from a holiday apartment in Praia da Luz, in Portugal’s Algarve 9 years ago.
The police did not take a statement from her and immediately ruled her sighting out, she said.
A man from Yorkshire also claimed he may have seen Maddie in Marrakech on the same day.
He said he saw her at the Hotel Ibis, which is next to the petrol station the woman believes she saw Maddie at.
Gerry and Kate McCann have been appealing for help to find Maddie for nearly 10 years
There have been many alleged sightings of Madeleine McCann
There were also other sightings in Zaio, a town in north Morocco, later that month and on June 15.
Britain's spy headquarters, GCHQ, intercepted calls from Morocco in June of the same year which referred to "the little blonde girl".
The calls also mentioned a ferry from Tarifa, the Spanish city a few hours' drive from the resort where Madeleine McCann went missing.
The petrol station witness said if the girl she saw was not Maddie then police should say who it was.
She said: "The police dismissed it."
Madeleine McCann sightings
Madeleine McCann 'tour' DISGUSTS Loose Women panel
Katie Hopkins: Madeleine McCann is never coming home
The witness added: "They said they had checked it out.
"If somebody wants to have a look again, I don't know, maybe they can do something.
"It can happen that people are found after 20 years and somebody knows something, for sure."
She described the girl she saw as wearing blue pyjamas with dark bottoms.
The girl had a patterned top, blonde hair, and had a sad expression on her face, she said.