Police have recovered female DNA from a leather strap found some metres away from Lawrence's bag.
The evidence was amongst one of 540 crates of material that has been at a south London police station for years. It was analysed in a fresh round of tests which could bring a new line of enquiry, thanks to the latest advances in forensic science.
The breakthrough is believed to be the first time that a woman has been connected with the scene of the murder in Eltham, south-east London.
Lawrence, who was 18 at the time and studying his A levels, was attacked and stabbed to death by a racist mob in 1993.
Gary Dobson, 37, and David Norris, 36, were jailed for life in 2012 after being found guilty of the attack.
A witness appeal will be made on Monday (12 September) by Detective Chief Inspector Chris Le Pere, which will be broadcast on the BBC's Crimewatch programme. It is unclear if police believe the unknown girl or woman is a suspect, accomplice of the gang, a friend or a witness.
Duwayne Brooks, one of Lawrence's friends who witnessed the murder and managed to escape the racist mob attack, has never suggested a female was involved.
Detective Chief Inspector Clive Driscoll and his team re-examined the crates of evidence gathered during the original botched investigation, codenamed Operation Fishpool. Driscoll published his autobiography in 2015, claiming that "certain people" within Scotland Yard were against new investigations into other suspects after the 2012 convictions.
According to the Sunday Times, a source close to the family said: "I hope and pray this could be a breakthrough for the family."
Police are determined to find others in the area who have not come forward or been traced, according to a Guardian report. Potential witnesses may have been fearful some police officers were in the pocket of Clifford Norris – the father of David Norris, who was eventually convicted of Lawrence's murder.
After sentencing the killers, Mr Justice Treacy said in January 2012: "The convictions of Gary Dobson and David Norris will not hopefully close the file on this matter. On the evidence before the court, there are still three or four other killers of Stephen Lawrence at large.
"Just as advances in science have brought two people to justice, I hope the Metropolitan Police will be alert to future lines of inquiry not only based on advances in science, but perhaps also from information from those who have been silent so far, whoever they might be."