Seat device with two side parts (110, 111), a backrest element, and a seat element which can be anchored to the side parts (110, 111) at least in an indirectly detachable manner, wherein, in the region of an end face which is close to the relevant backrest element, the two side parts (110, 111) are respectively mounted about a first hinge, which can be pivoted about a perpendicular axis, in order to reversibly transfer the side parts (110, 111) from a normal position, in which the side parts (110, 111) are arranged perpendicularly to the backrest element, into a stowage position, in which the side parts (110, 111) are aligned parallel to the backrest element, and the side parts (110, 111) are respectively subdivided into a subregion (110', 111') which is close to the backrest element and a subregion (110", 111"') which is remote from the backrest element, wherein, in the region of an end face which is close to the relevant backrest element, a respective subregion (110', 111') which is close to the backrest element is mounted about the first hinge, which can be pivoted about a perpendicular axis, and wherein, in the region of opposite end faces, the two subregions (110', 111', 110", 111"') are respectively mounted about a second hinge, which can be pivoted about a perpendicular axis, in order to reversibly transfer the respective subregions (110', 111', 110", 111"') from a collapsed state, in which the subregions (110', 111', 110", 111"') are arranged substantially parallel to the backrest element, into an expanded state, in which the respective two subregions (110', 111', 110", 111"') in the normal position are arranged perpendicularly to the backrest element, and wherein furthermore provided in the region of an end face, remote from the backrest element, of a subregion (110", 111"), respectively remote from the backrest element, of a respective side part (110, 111) is a third hinge with a perpendicular pivot axis for fastening a connecting part which connects the two side parts (110", 111"), wherein the seat element can be reversibly transferred from a collapsed state, in which the seat element is arranged parallel to the backrest element, into an expanded state, in which the seat element is arranged perpendicularly to the backrest element and perpendicularly to the side parts (110, 111), wherein there is provided a connecting strut, which connects the connecting part to the backrest element and is subdivided by a central pivot hinge into a first part, which is close to the backrest element, and a second part, which is close to the connecting part, wherein an end face of the first part that is close to the backrest element is articulated on the backrest element by means of a first pivot hinge and an end face of the second part that is close to the connecting part is articulated on the connecting part by means of a second pivot hinge, characterized in that the first pivot hinge is arranged above the second pivot hinge, and the first part and the second part of the connecting strut as a whole have a greater length than the maximum distance between the backrest element and the connecting part, with the result that, when there is the maximum spacing between the backrest element and the connecting part, for the purpose of a clearly predefined direction for folding the connecting strut about the central pivot axis in the course of transferring the seat device from a normal position into a stowage position, the first part and the second part of the connecting strut are at a predefined angle of less than 180° with respect to one another.