The 10-person Gloucester agency, founded by Rich Leigh, has pledged that it will not cut salaries as part of the move.
The four-day plan will initially run on a trial basis for six weeks starting Monday 25 July, after which a decision will be made on whether to make the arrangement permanent.
Leigh says in a post on the agency website: "It started with me thinking about us already finishing at 4pm on Fridays, wondering how we’d fare if we went down to a half day, and has gone from there."
Leigh, who in 2016 legally changed his name to Mr Public Relations, continues: "How do you get happy clients? Great work. How do you get great work? A happy team. Technology was supposed to give us a better work-life balance. If anything, it’s made it worse.
"In a broader context, I think that getting rewarded for being good at your job has been replaced by a culture that celebrates being wedded to your job, above all else. Presenteeism is good for nothing and nobody."
Radioactive is not the first UK marcoms firm to make this change - a London design company and a Glasgow marketing agency were profiled by the BBC earlier this year for making the switch.
Other PR firms have taken the approach of introducing unlimited annual leave policies - with mixed results.
Leigh's post also says that reaction from clients had been "100 per cent positive, so far" and notes: "Clients do not pay us to work on specific days and nor has one single client ever dictated the days we work – that’s for me and us to decide, based on each client and our workload."
It concludes: "It's either a spectacularly daft idea, or... it just might work."