The agency will provide strategic advice, content creation, social media support, web design, and media training to sport, consumer and luxury businesses in Oxford, London and the Thames Valley region.
Clients already on Fortitude's roster include bespoke gifts brand Thomas Lyte, The Oxford Bus Company and Holiday Rewards.
On setting up the agency in Oxford, Box Turnbull said: "There are so many excellent businesses in Oxford, with great stories to tell, a lot of which will not have benefitted from serious strategic PR support to date.
"I do believe that businesses are always seeking a more joined-up approach and with the huge opportunity that now exists in owned media, dovetailing digital design flair and brand positioning into strategic communications makes sense."
Box Turnbull has launched the agency alongside digital creative Liam Faulkner, who is its creative director.
Prior to founding Fortitude, Box Turnbull was managing director at Oxford United FC between 2016 and 2017. Before this he was managing partner at Mayfair-based PR agency Dragon Advisory, where he provided strategic counsel and commercial guidance to clients.
Between 2004 and 2012, Box Turnbull was a journalist at the Daily Mirror. He was one of multiple journalists arrested in connection with Operation Elvenden - an investigation into alleged illegal payments made to police and other public officials.
Box Turnbull was cleared of any wrongdoing in 2015.