Bryant, who joins after six years at rival developer Blizzard, will oversee Jagex's global PR strategy and head up a team of two, reporting to Mark Gerhard, CEO and chief technology officer of the company.
Bryant has been tasked with the upcoming commercial launch of the game Transformers Universe, which entered open beta earlier this summer. He will also lead the PR launch of an expansion pack for popular online game RuneScape and a third title that is yet to be announced.
Jagex works with a roster of agencies across the globe, including Kazoo PR in the UK.
Bryant, who has also worked at Vivendi Universal Games and Take 2 Interactive, said: "The entire Jagex team overflows with talent, enthusiasm and passion, and securing a role with them at any time would have made for a wonderful opportunity."
Gerhard said: "It’s a tremendous pleasure to welcome Jonnie to Jagex from both a personal and professional perspective.
"His significant experience of working on MMOs [massively multiplayer online games], strategy and fantasy games, and more, couldn’t come at a better time as our studio gears up to publish Transformers Universe."