The broadband and telecoms company is giving away the money between now and the end of August in areas including Leicester Square, Oxford Circus, Covent Garden, Westminster, Trafalgar Square and the South Bank, as well as on the Tube network.
Other UK cities will have the chance to be "put pocketed" later in the year.
TalkTalk has assembled a team of what it calls 20 "ex-pickpockets" to execute the stunt.
You can read a post on the TalkTalk Blog about it here.
The campaign has met with a mixed response from those who like the idea of finding a fiver in their trousers, to those who have been victims of bag snatching and other street crimes questioning the sense of such a campaign.
However, TalkTalk has anticipated such fears and says that the Metropolitan Police have been notified of the campaign.
The ex-crims have been given ID and minders in case they do get caught in the act although TalkTalk said that the stunt has been in pilot since July 1 and none of the team has yet been rumbled.
Mark Schmid, communications director of TalkTalk, said: "With so many scams out there, Britons have become very sceptical of companies giving money away -- so we have turned to Put-Pocketing to give something back.
"While unconventional, we don't think anyone is going to mind finding a crisp £20 in their pocket courtesy of the activity."
UPDATE: TalkTalk has confirmed that lucky recipients of put pocketing cash will receive a branded card along with the cash.
The campaign is being promoted with posters at relevant locations proclaiming "Rejoice! Put-pockets operating in this area."