He will initially provide insight and expertise on a wide range of accounts across all divisions of PHA Media, with a particular focus on high-end clients and online reputation management.
Clench joined The Sun as a reporter in 2000 and held a number of roles including royal correspondent and most recently head of news. According to LinkedIn, he left the newspaper in December 2015.
During his time at the national newspaper, he covered topics from school shootings in the US to the war in the Middle East and published a book on the romance between Prince William and Kate Middleton, now the Duchess of Cambridge.
In a statement, Clench said he was excited to join a "well respected" agency.
"I’ve been impressed by the culture of innovation and expertise at the firm, which has clearly been integral in attracting so many top clients in recent years," he said.
Chairman and founder Phil Hall said: "James’s vast and varied journalistic experience and knowledge of the media will be invaluable to our clients here at PHA Media."
Clench is the latest high-profile journalist to move into the world of comms. In February, former Independent editor Chris Blackhurst joined CrosbyTextor and at the beginning of March, The Times’ deputy business editor Andrew Clark took on a role as director in the public affairs team at Burson-Marsteller.