The brief includes raising awareness and driving sign-ups for the obstacle course event, which The PHA Groups says is the first of its kind to be held at a major sports venue inside a UK sports stadium.
The PHA Group will also assist Spartan Race with the rest of its UK events throughout the year.
Spartan Race was founded by adventure racer Joe De Sena in 2008 and has grown to become the world’s largest obstacle course race and endurance brand in the world, with one million racers taking part in more than 200 events in 42 countries each year.
Spartan Race UK general manager Sam Lansdale said: "2019 is an exciting time for Spartan Race with the launch of our Spartan Stadion race at the iconic Twickenham Stadium. The venue is steeped in history and this will be the first ever obstacle course race to be held there.
"The PHA Group have vast experience promoting mass participation running and obstacle course racing events and the company’s knowledge of the media landscape will hugely benefit Spartan."
The PHA Group associate director and head of sport and fitness, Tom Inskip, said: "The Spartan Stadion Race is a unique concept that is going to give racers the opportunity to race in one of the most iconic stadiums in the country."