An extendable tablet includes a tabletop frame including two sliding tabletops, a foldable tabletop having a first folding panel and a second folding panel pivotally coupled with each other, two sliding runners mounted underneath the two sliding tabletops to allow the two sliding tabletops to slide between a full-size position and a compact-size position, and a folding joint mounted to one of the sliding runners to pivotally connect to the first folding panel of the foldable tabletop. At the full-size position, the first and second foldable panels are pivotally folded between the two sliding tabletops to maximize a utilizing area. At the compact-size position, the first folding panel is pivotally overlapped on the second folding panel at a position that the foldable tabletop is sunk underneath the sliding tabletops such that the sliding tabletops are slidably mounted with each other edge-to-edge to minimize the utilizing area.