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 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 opposite to each other; - said support element is arranged alongside one of said side surfaces 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 (C); - 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 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 opposite to each other; - said support element is arranged alongside one of said side surfaces 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 (C); - said first end portion defines said shoulder portion . <EOS>