Abstract: The invention relates to a trim assembly (1) of a motor vehicle, comprising a passenger compartment mat (3) extending between a visible face (4) and a reverse face (5), the mat (3) comprising a side contour surface (6) that joins said faces, the assembly (1) comprising a soundproofing element (7a), which is designed to occupy the space (8a) comprised between a floor (9a) of the passenger compartment (2) of the vehicle and the reverse side (5) when the mat (3) is installed in the vehicle, the assembly (1) comprising at least one trim element (13) designed to at least partially conceal a side rail (10a) of the vehicle. The mat (3) is designed to be positioned in the passenger compartment (2) of the vehicle with the contour surface (6) positioned opposite at least a portion of the side rail (10a), the trim element (13) being designed to conceal, when it is installed on the side rail (10a), a portion of the visible face (4) of the mat (3).