Crisis said the messages, which it dubbed reverse graffiti, "clean up" a dirty section of wall with words that form the image of a huddled homeless person.
The copy reads "Most homeless people have moved on but their problems haven't gone away" and includes the Crisis address.
Fifteen outdoor locations around London have been marked with the image.
Crisis said that despite fewer people now living on the streets, there was still a sizeable hidden homeless population of thousands "living invisible lives" in hostels and other temporary accommodation.
The charity said the campaign would aim to help people find a route out of homelessness and rebuild their lives by finding them affordable housing, training and grants to begin a career.
The campaign is the latest WWAV Rapp Collins London has done for Crisis, and follows the agency's national press inserts work and digital marketing through MySpace, Facebook and the charity's microsite.
Andrew Page, director of fundraising at Crisis, said: "This campaign seeks to highlight that, while there are less visible signs of homelessness, the problem remains a major one and it's imperative we get public backing that will allow us to continue to offer the help and support that can literally lead to many men and women rebuilding their lives."