CANNES, France – While Snapchat had its Ferris wheel and Google and Facebook had their beach setups, Reddit was low-key at the Cannes Lions ad festival this year, renting a modest apartment near the action on the busy coastline.

But the high-trafficked website did come with a fresh pitch for advertisers. It is rolling out video ads, which will be available through its sales team and its existing self-service ad buying platform.

“It’s...