She arrives at PHA following two years advising the Deputy Prime Minister, working closely with chief of staff Jonny Oates on strategy and negotiating within the coalition.
Before this, Foster was CEO of the Welsh Liberal Democrats for four years, running four national campaigns for the party.
PHA Media founder Phil Hall said that an evolving client base, including national education charity Young Enterprise, had led to an increasing demand for such expertise.
"It’s a natural progression for us and we have a lot of companies, as well as some political individuals and countries, who want to understand the political set-up.
"Jo is a very confident woman with a strong view on things. She gets on with things, finds solutions to problems and is very dynamic and on the ball – that’s why she’s been advising at the highest level."
Foster will work alongside the agency's strategic comms team, which was set up last month and is led by PHA’s Neil McLeod and Ciaran McCale.
Last month she was named among The Telegraph's 50 most influential Liberal Democrats.