The woman said something to one of the men which convinced the McCanns' private investigators she might have information about Madeleine.
The sketch was issued by McCann spokesman Clarence Mitchell and David Edgar, the retired detective who now leads the search for the missing girl.
The pair gave details of the development at a press conference in London.
Mitchell said the two men first saw the woman when they were drinking in bars at the city's Port Olimpic Marina at about 2 a.m.
He said she was well-dressed but appeared agitated and kept walking up and down outside the El Ray de la Gamba restaurant bar.
One of the witnesses, a 41-year-old British man, had a brief conversation with her.
Mitchell would not reveal what she said for operational reasons, but said it was "potentially significant" to the little girl's disappearance.
He said the woman was then seen having a "heated conversation" with another person in Spanish in a nearby bar. She then wandered off towards a hospital.
The witnesses said the brown-haired woman was aged between 30 and 35, slim, about 5 feet 2 inches tall, and described as "a bit of a Victoria Beckham lookalike."
She is said to have had an Australian accent, and is also thought to speak fluent Spanish or Catalan.
Edgar said the witnesses had not come forward earlier for "personal reasons" and denied that they were drunk, even though they had visited a number of bars on that night.
He said the conversation took place about 72 hours after Madeleine disappeared — and it was possible that Madeleine could have been taken to Barcelona by boat in that time.
Mitchell said Madeleine's parents Gerry and Kate "were not surprised" when told about the woman but were not "getting their hopes up or down."
But he added: "In this case they are hoping this might prove as significant as Dave believes."