The Calm Belt is an area of ocean that borders the north and south of the Grand Line. It is a very effective barrier for those trying to enter the Grand Line, as it is always completely calm and never has any ocean currents or winds blowing, making it impossible for non-self-propelled ships to travel through. The Calm Belt is also a nesting ground for the worlds largest Sea Kings.

Although it is difficult to traverse the Calm Belt, there are ways to do so. Ships that can sail without wind, such as Boa Hancocks ship that is pulled by Sea Kings, can travel through the Calm Belt. There are also plenty of Devil Fruit users that have the ability to get across, such as Kuzan, who can glide across the water, or even Buffalo that can fly over. However, the Calm Belt is still a deterrent to rookie pirates and not much else.