The smallest turtle in the world can be broken down into a couple of different answers. The group Chelonians, a subdivision of reptiles includes turtles, tortoises and terrapins. The smallest Chelonian in the world is the Speckled Cape Tortoise found in Western South Africa; it grows between 6 and 8 centimeters and the males are smaller than the female. These creatures are marked Near Threatened by the IUCN. One reason is that the females lay only one egg each summer; which is hardly enough to restore the population that is dying. Other than that, scientists know very little about their mating.
Here is a little brief about the difference between turtles, tortoises and terrapins. Turtles are mostly aquatic and often have special webbed toes to allow them to move easily through the water. They are air breathing, but hold their breath when they go underwater. Tortoises are mostly land dwelling and keep a supply of water in their shell. Turtles have a comparatively flatter back compared with tortoises, which have a much more rounded form. Terrapins are between the two; they live partly on land and partly in the water and usually refer to a creature whose characteristics are between a turtle and a tortoise.
That being said, the smallest actual turtle in the world is the Bog Turtle, native to the Eastern United States. It grows to be about 11 centimeters long. Like their name implies, these turtles live in sphagnum bogs, wetlands and wet pastures. They like environments that are calcium rich; calcium carbonate, calcium or lime. They are listed as Threatened by the IUCN, mostly due to habitat destruction. As a result, some states have banned the removal of these turtles from their environments.