Abstract: Insertable armrest frame and mounting structure is designed so that an upholstered, pivotable armrest can be easily attached to an upholstered seat back in such a manner that the mounting elements will be concealed. The mounting and support structure is quite light in weight but very strong due to use of an arm frame member which incorporates at least one strengthening rib portion. The arm frame pivots on one pivot pin anchored to the seat back frame and, in a preferred embodiment, includes a strengthening rib incorporating a cup-shaped portion having a curved flat rim which is slightly resiliently deformed by a flanged retaining nut on a stop pin which is also anchored in the seat back frame. The resilient deformation provides uniform tension which tends to hold the arm in any position to which it is rotated.