As well as giving PR advice, it will be encouraging its employees to volunteer for the charity.
'With the charity's 40th anniversary approaching there's an important job to be done in helping increase the charity's profile and ultimately raise vital funds,' said Red's CEO Mike Morgan.
Red will particularly target London media, because the charity is based in Soho. It will also attempt to get more celebrity supporters as well as educating local businesses about the charity.
Red's first big media push will be the Sleep Out, one of its biggest fundraising events where fundraisers sleep rough for a night. A Christmas ad campaign launches on the 16 November.
The agency will report into COO Seyi Obakin. Centrepoint stopped working with Redhead PR earlier this year.