The 24% reduction has been made to ensure "the highest standard of meaningful debate can take place", the festival’s organisers said. Last year there were 387 members on juries from 40 countries.
The first phase of the voting process for the Promo & Activation, Media and Direct Lions will now be completed in the weeks before the festival in June by a shortlist jury.
Jury panel sizes will also be reduced across the Film, Radio, Print & Publishing, Outdoor, Digital Craft, Creative Effectiveness and Mobile categories.
Jose Papa, the managing director of Cannes Lions, said some juries have grown because of an increase in the number of entries they have to judge.
The Titanium Lions, which celebrate the best in innovative and game-changing work from around the world, will have a maximum of ten jurors, described by Papa as "the gold standard".
"After extensive consultation with previous jurors and the wider industry, we have concluded that we can reduce the number of jury members while still allowing them plenty of time to judge work," he added.
Sir John Hegarty, the Bartle Bogle Hegarty co-founder and jury president of the Titanium category this year, said: "I would rather be judged by fewer jurors who are more focused, more responsible and better qualified.
"Collective responsibility and the quality of discussion are lost if you have too many judges. Size is no guarantee of strength."