Impact, which specialises in financial PR for the media sector, will handle a remit to build SEM's corporate profile following its flotation last August and rebrand from Jerome Anderson Management.
The agency team, which is led by Impact account director Jane Wilson, will target the media, sports business and financial community, focusing on analysts.
'SEM is looking to change how the company is perceived among the investment community and position itself as the authority on the sports business,' said Wilson.
'Part of our job is to make sure the investment community understands how SEM works and the type of contracts it has with football clubs, for example.'
Impact first worked with SEM on a short-term project, handling the controversial press conference held earlier this year to publicise the Lennox Lewis versus Mike Tyson fight, which was close to being called off due to a brawl between the two boxers.
The agency, headed by managing director Simon Forrest, won the brief after a non-competitive pitch.
SEM manages the interests of 175 football-related clients across the UK and Europe and around 75 footballers, such as French Premiership stars Thierry Henry and Emmanuel Petit.
The firm is also developing an entertainment division, with clients so far including former Arsenal players Charlie Nicholas and Ian Wright.
It was founded in 1984 by Jerome Anderson.