A modular upholstered piece of furniture, formed by a plurality of modular compositions that are substantially composed of a polymeric material, wherein the modular compositions are associated to each other through at least one fitting element arranged on each of the modular compositions, so that the different modular compositions have fitting elements of cooperating structural shape to provide association of the modular compositions and shaping of the modular piece of furniture wherein the fitting elements are configured as at least one from the following elements: press elements, slide elements, connection crossbars and connection pins, further the modular compositions comprising: - a set of support bases, at least one pair of seat guides and at least one pair of side rests, wherein - said set of support bases is provided with a pair of slide protrusions to provide association of each of the support bases with said at least one pair of seat guides or said at least one pair of side rests, wherein: - the seat guides comprise slide courses for association with the slide protrusions of the support base, further: - the pair of side rests is also provided with slide courses for association with the slide protrusions of the support bases, wherein the slide protrusions are arranged along the sides of smaller dimension of the support bases, said slide protrusions being configured as shoulders from the side ends of the support base, wherein the slide courses are arranged in a portion of the seat guides and of the side rests that faces an inner area of the modular piece of furniture, each slide course being provided with a track suitable for engagement and displacement of the slide protrusions, wherein the slide courses are configured as cutouts in the portion of the seat guides that faces the inner area of the modular piece of furniture, wherein the slide courses extending along substantially the whole height of the seat guide, characterized in that the slide courses further comprising a plurality of catches, said catches establishing widths for the slide course slightly smaller than its total width, and that the modular piece of furniture further comprises a set of support brackets, wherein each of the support brackets is associable to at least one of the side rests. - a set of support bases, at least one pair of seat guides and at least one pair of side rests, - said set of support bases is provided with a pair of slide protrusions to provide association of each of the support bases with said at least one pair of seat guides or said at least one pair of side rests, wherein: - the seat guides comprise slide courses for association with the slide protrusions of the support base, further: - the pair of side rests is also provided with slide courses for association with the slide protrusions of the support bases, the slide protrusions are arranged along the sides of smaller dimension of the support bases, said slide protrusions being configured as shoulders from the side ends of the support base, wherein the slide courses are arranged in a portion of the seat guides and of the side rests that faces an inner area of the modular piece of furniture, each slide course being provided with a track suitable for engagement and displacement of the slide protrusions, wherein the slide courses are configured as cutouts in the portion of the seat guides that faces the inner area of the modular piece of furniture, wherein the slide courses extending along substantially the whole height of the seat guide, characterized in that the slide courses further comprising a plurality of catches, said catches establishing widths for the slide course slightly smaller than its total width, and that the modular piece of furniture further comprises a set of support brackets, wherein each of the support brackets is associable to at least one of the side rests.