The Wing's second screen can either act independently of the larger screen while it is displaying a film, for example, or can complement it as a controller for gaming and media.
However, some of this functionality relies on using pre-installed browsers.
One expert said it was one of the "more radical designs" he had seen.
"It's a very tricky design to evaluate without using it - it is reminiscent of some of the designs we saw from Japan back in the late 2000s," Ben Wood from the consultancy CCS Insight said, noting it was unlike any of the folding-screen designs that several of LG's rivals have recently focused on.
"You have to applaud LG for experimenting. Ultimately it will be up to consumers to decide whether its an approach that works."
The 5G device also has three cameras on the back and a fourth pop-up lens.
LG said the Wing phone would initially launch in South Korea next month.
North America and European launches would follow, it added, but the handset price has yet to be revealed.