An adjustable support arm assembly includes a holder frame affixed to a support, a mounting frame for supporting an object, and a coupling structure coupled between the holder frame and the mounting frame. The coupling structure comprises a female coupling member, a male coupling member inserted into the female coupling member, a polygonal shaft with a peripherally toothed head inserted through the female coupling member and the male coupling member and movable between a locking position to lock the female coupling member to the male coupling member and an unlocking position for allowing turning of the female coupling member about the polygonal shaft relative to the male coupling member, and a compression spring that supports the polygonal shaft in the locking position.