A desert is a region that is simply very dry because its receives little to no water. To be known as a desert, an area must receive less than 250 millimeters of annual precipitation. Roughly 1/3 of Earth's land surface is a desert.
If you consider all the types of desert, then it's Antarctica which is 14 million square km. it is a cold desert. But when you talk about largest hot desert, its Sahara Desert.
The world’s largest desert is the Sahara Desert, a subtropical desert located in northern Africa. It covers a surface area of about 3.5 million square miles. It is one of the driest and hottest regions of the world. It is the largest non-polar dessert. The surface of the land is constantly being reformed by winds and the extremely low rainfall.
Note: Most people think that Sahara is the largest dessert, but it’s the largest hot dessert. The largest dessert is Antarctica followed by the Arctic which is cold dessert but known as the largest dessert.