Cushion for seat

A cushion for a seat, including a foam padding integral with a three-dimensional cover (1) covering the padding, the three-dimensional cover including a first piece (10), a second piece (12), the second piece having first and second spaced apart sections (12b,12c), and a connecting section (12a), a peripheral piece (11), an intermediate piece (13), a third piece (14), slide fastener devices, a first engaging device attached to a free end of the third piece (14), a second engaging device attached to a free end of the intermediate piece (13), and third engaging devices attached to free ends of the first and second sections (12b,12c) of the second piece (12).