She will leave in around eight weeks to set up Plank Public Relations, working for clients including ITV newsreader Andrea Catherwood (formerly at Five), independent production firm MAP TV - and Five.
Plank, who is head of factual and features publicity, said: 'I've thoroughly enjoyed the last seven and a half years at Five. I feel the time is right to make this move and I'm delighted I'll still be working for Five.'
Plank joined the channel in 1998, a year after it launched, and took up her current role in 2001. Five head of press Paul Leather said Plank had played a 'key role in Five's success'.
Campaigns she worked on include promoting Talk to the Prime Minister in February, the relaunch of Five News in January, its Alastair Campbell Interviews... series in 2004, and the return of newsreader Kirsty Young.
She headed publicity for both series of The Farm, which gained tabloid front-page notoriety last year when a contestant, Rebecca Loos, masturbated a boar.
Leather is expected to kick off a recruitment process for Plank's successor shortly. Leather runs a 17-strong PR team at the channel, working alongside deputy head of press Tracey O'Connor.
Prior to Five, Plank worked at Golley Slater Public Relations.
Her future projects for Five are yet to be confirmed.