Seat for a motor vehicle, comprising: - a backrest; - a seat portion comprising a fixed element and at least one movable element which comprises a body having an upper surface and a pair of side surfaces (35, 36) opposite to each other; said body being displaceable between: a) a lowered configuration, in which said upper surface forms, with an upper face of said fixed element and with said backrest, a place for an adult passenger; b) a raised configuration, in which said body is arranged higher than when it is arranged in the lowered configuration and forms a booster seat suitable for supporting a child; and - fastening means for releasably fastening said body at least in the raised configuration; said movable element further comprising at least one side support element, which comprises a shoulder portion and is displaceable with respect to said body between: - an operating configuration, in which said support element is arranged next to said body and said shoulder portion projects upwards with respect to said upper surface, at least when said body is in the raised configuration, to limit, in use, any side displacement of a child seated on said booster seat; and - a rest configuration, in which said support element is arranged lower or flush with respect to said upper surface, at least when said body is in the lowered configuration; characterized in that: - said support element comprises an arm having a first and a second end portion (66, 67) opposite to each other; - said support element is arranged alongside one of said side surfaces (35, 36) when said support element is in the rest configuration; - said movable element further comprises a coupling device, which couples said second end portion to said body in a rotatable manner about a hinge axis; - said first end portion defines said shoulder portion. - a backrest; - a seat portion comprising a fixed element and at least one movable element which comprises a body having an upper surface and a pair of side surfaces (35, 36) opposite to each other; said body being displaceable between: a) a lowered configuration, in which said upper surface forms, with an upper face of said fixed element and with said backrest, a place for an adult passenger; b) a raised configuration, in which said body is arranged higher than when it is arranged in the lowered configuration and forms a booster seat suitable for supporting a child; and a) a lowered configuration, in which said upper surface forms, with an upper face of said fixed element and with said backrest, a place for an adult passenger; b) a raised configuration, in which said body is arranged higher than when it is arranged in the lowered configuration and forms a booster seat suitable for supporting a child; and - fastening means for releasably fastening said body at least in the raised configuration; said movable element further comprising at least one side support element, which comprises a shoulder portion and is displaceable with respect to said body between: - an operating configuration, in which said support element is arranged next to said body and said shoulder portion projects upwards with respect to said upper surface, at least when said body is in the raised configuration, to limit, in use, any side displacement of a child seated on said booster seat; and - a rest configuration, in which said support element is arranged lower or flush with respect to said upper surface, at least when said body is in the lowered configuration; characterized in that: - said support element comprises an arm having a first and a second end portion (66, 67) opposite to each other; - said support element is arranged alongside one of said side surfaces (35, 36) when said support element is in the rest configuration; - said movable element further comprises a coupling device, which couples said second end portion to said body in a rotatable manner about a hinge axis; - said first end portion defines said shoulder portion.