Ascential is tasking the agency with supporting the existing Cannes Lions press office team by spotlighting specific messages and themes around the festival to international media and journalists.
The Festival Press Centre welcomed 767 accredited journalists in 2016.
Propeller founder Martin Loat said: "The Propeller team have attended Cannes Lions for several years. As a specialist in business PR for the advertising, media and tech industry, we know the terrain and we're well-suited to fulfilling the brief set for us by Ascential."
There was no competitive pitch for the account. Work began this week on the brief, and will continue until mid-July. Ascential is not believed to contract any other PR agency to support the Lions specifically.
The festival itself runs from 17 to 24 June this year. Lions will be handed out in a total of 24 categories, including the PR Lions.
Cannes Lions welcomes more than 15,000 attendees from 90 countries.
After growing its revenue by 14 per cent to £1.9m in 2016, Propeller sits at 123rd in the new PRWeek UK Top 150. Other clients of the ad tech, media and digital-focused PR firm include Google, JCDecaux, the Daily Mirror and SapientNitro.
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