Jackson, currently director of media content at creative consultancies @TheMoment and @TheMinute, was previously an editorial consultant at Freuds between 2015 and 2016, his LinkedIn profile states.
He has rejoined Freuds as an associate director in the agency's 62-person corporate division, The Brewery, and will report to the division's MD Arlo Brady.
Freuds director Ruth Settle told PRWeek the agency is "always looking to hire talented people", adding that Jackson's appointment marked the first of "several new high-profile" hires the agency would be annoucing in the coming months.
Having joined The Sun in 1999 as features editor, Jackson was appointed as the newspaper's environment editor in 2009, covering global energy and environment news. Following this, he provided editorial services to the Sustainable Innovation Forum, which was part of the 2015 Climate Change Conference in Paris (COP 21).
Brady said an increasing number of the agency's clients were demanding "deep expertise" on sustainability and purpose, adding that the appointment of Jackson "strengthens our offer in this space and increases our impact on the issues we care about".
Freud Communications, founded by PRWeek UK Hall of Fame inductee Matthew Freud, grew 29 per cent by revenue last year, according to the Top 150 agency rankings 2017.