A support for an armrest of a motor vehicle seat, formed of generally U-shaped solid plastic defining: a bottom intended to be attached to an armature of a backrest of the seat, and two parallel branches perpendicular to the bottom, each branch defining in a surface opposite to the other branch a hole intended to receive one end of an axis of the armrest and a groove intended to receive an armrest rotation stop element. An armrest armature, an armrest and its assembly method and a motor vehicle seat.