The account, which begins this month, was awarded to Hudson Sandler without a competitive pitch.
PTSG, which was founded in 2007, employs more than 220 people across nine UK sites and listed on the London Stock Exchange's AIM market in February.
According to a Hudson Sandler press release, the company has "the objective of building the UK’s leading niche specialist services provider through a combination of acquisition and organic growth".
Paul Teasdale, CEO of PTSG, said: "In Hudson Sandler we have found an experienced partner to match our ambitious growth strategy. We are excited about the next phase in the company's development and to be working with Hudson Sandler to deliver our investment case."
Cat Valentine, a director of the agency who leads the account, said: "We thoroughly enjoy working for companies with a clear vision and strong growth strategy and are very much looking forward to advising PTSG."