The object of the invention is a horizontal paddle dryer that is used for normal or vacuum drying of soaked substances that are in the form of dry powder mixtures or granulates in a manufacturing process and are soaked and mixed (normally with solvents) due to technological needs. Once drying is completed, the interior of the dryer needs to be washed due to various types of substances that differ from each other in content and properties. The construction of the horizontal paddle dryer of the invention allows simple and effective washing of the horizontal paddle dryer without an additional removal of washing heads during the washing of the dryer. The horizontal paddle dryer of the invention has a specially shaped interior wall (1) of the drum (2) that passes on the upper part through two surfaces (1a, 1b) that extend tangentially to the interior wall (1) into a horizontal final surface (1c) of the internal wall (1), such that a slightly concave part is created in the interior of the drum (2) in the wall (1), in which concave part retractable washing heads (3) are arranged that do not hinder functioning of the rotor (4) with paddles (5) either during drying or during washing.