The Hard Calls Save Lives campaign, which runs until the end of the year, is aimed at mothers, sisters and aunts in areas most affected by street violence.
Police are interested in anyone carrying a knife or where a weapon is hidden.
The appeal comes as police investigate the murder of a 15-year-old boy stabbed to death in Wandsworth on Thursday – the 13th teenager victim this year.
Alexa Loukas, Crimestoppers’ London regional manager, said: “We particularly want to inspire women who may have a close relationship to someone on the periphery of knife crime – mums, sisters and aunties.
“The role of the public in speaking up about knife crime cannot be underestimated – one phone call could literally stop another young person being killed.
“It can feel like a hard call, but it could save the life of someone’s son or daughter.
“Have you seen drug deals taking place in the area? Are cars arriving at unusual times of the day and night?
“The purpose of this knife crime campaign is to inform and inspire the community and encourage people to come forward and share information anonymously.
“If you have any information about knife crime, please get in touch – your anonymity is guaranteed.”
Last year, Crimestoppers passed on 14,000 intelligence reports to the Metropolitan Police. Appeals for London’s most wanted criminals led to 45 arrests.
The charity guarantees anonymity and callers will not be asked to reveal their name, gender, age or ethnicity. No calls are ever recorded and you don’t need to go to court.
Contact Crimestoppers on freephone 0800 555 111 or the anonymous online form at Crimestoppers-uk.org